How to convert HTML form to C# for PayPal subscription - asp.net-mvc

Question

I have the following HTML which works well for subscription payments in my test environment.
<form action="paypalWebAddress" method="post" target="_top">
<input type="hidden" name="cmd" value="_xclick-subscriptions"/>
<input type="hidden" name="business" value="myPaypalAccountId"/>
<input type="hidden" name="paymentaction" value="sale"/>
<input type="hidden" name="item_name" value="Subscription For abc123"/>
<input type="hidden" name="currency_code" value="GBP"/>
<input type="hidden" name="no_note" value="1" />
<input type="hidden" name="a3" value="50"/>
<input type="hidden" name="p3" value="6"/>
<input type="hidden" name="t3" value="M"/>
<input type="hidden" name="src" value="1" />
<input type="hidden" name="srt" value="0" /><!--needs to be never ending-->
<input type="hidden" name="return" value="www."/>
<input type="hidden" name="notify_url" value="www."/>
<input type="hidden" name="cancel_url" value="www."/>
<input type="hidden" name="custom" value="abc123"/>
<input TYPE="hidden" name="charset" value="utf-8">
<input type="hidden" name="bn" value="Me_Subscribe_WPS_UK" />
<input type="submit" name="submit" value="Make Payment"/>
</form>
I'm about to go live and concerned that some one could easily manipulate the DOM and I'm suddenly sending bogus payments. I feel it would be safer to somehow post these values from the code behind so the user can't tamper with the values but, the user will still need to use the PayPal interface in their browser to log in and subscribe.
PayPal-NET-SDK has a quick start and shows how to get going. I have completed this (copied and pasted and ran in debug mode) and it executes fine. However, this doesn't cover subscriptions, probably for the reason explained above (and would probably prevent them accessing the PayPal gui in their browser).
I get the option in the PayPal dashboard area to create a button and reference by button, but, this won't work for me as I need to be able to set the custom field per subscription as a user can subscribe to 1 or more services and each service is unique to that user.
I can't understand how to achieve this

Answer 1

If I understand this right, every subscription is unique per customer in your case and that won't work with the static buttons, so the API seems the only way to go.
The PayPal API does cover subscriptions, they're called Billing Plans. Because each subscription is different, you could create a new Billing Plan for each unique service (or combination of services). This can be done without any intermediate SDK using the HttpClient directly to their API.
Alternatively, the PayPal-NET-SDK you mentioned also seems to cover Billing plans. Their Sample application has quite a lot of examples on how to create, enumerate and issue Billing Plans. Make sure you check out the billing_agreement_tokens collection on the Payment class. You might add multiple "service" tokens there.
No matter how close you are to releasing, a lot of testing still seems to be in order here.

Answer 2

You can try using HttpClient, something like this will help
HttpContent content = new FormUrlEncodedContent(
new List<KeyValuePair<string, string>> {
new KeyValuePair<string, string>("key1",
"value1"),
new KeyValuePair<string,string>("key2","value2")
});
content.Headers.ContentType = new
MediaTypeHeaderValue("application/x-www-form-
urlencoded");
content.Headers.ContentType.CharSet = "UTF-8";
HttpResponseMessage resposne = await
client.PostAsync(new Uri("paypalWebAddress"),
content);

Answer 3

You should not hardcode the values in forms, instead you should create an API in the backend which will return the correspoding values.Then you can dynamically create the form based on the response of the API.
Make sure you create this form after you have received the response from the API.You can have a check like if response is null show some spinner.
<form action="paypalWebAddress" method="post" target="_top">
<input type="hidden" name="cmd" value="{response.cmd}"/>
<input type="hidden" name="business" value={response.business}"/>
<input type="hidden" name="paymentaction" value={response.action}/>
<input type="hidden" name="item_name" value="{response.item}"/>
<input type="submit" name="submit" value="Make Payment"/>
</form>

Answer 4

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="business.test.com">
<input type="hidden" name="item_name" value="Memory Stick">
<input type="hidden" name="item_number" value="MEM32507725">
<input type="hidden" name="amount" value="3">
<input type="hidden" name="tax" value="1">
<input type="hidden" name="quantity" value="1">
<input type="hidden" name="currency_code" value="USD">
<!-- Enable override of buyers's address stored with PayPal . -->
<input type="hidden" name="address_override" value="1">
<!-- Set variables that override the address stored with PayPal. -->
<input type="hidden" name="first_name" value="John">
<input type="hidden" name="last_name" value="Doe">
<input type="hidden" name="address1" value="345 Lark Ave">
<input type="hidden" name="city" value="San Jose">
<input type="hidden" name="state" value="CA">
<input type="hidden" name="zip" value="95121">
<input type="hidden" name="country" value="US">
<input type="image" name="submit"
src="https://www.paypalobjects.com/en_US/i/btn/btn_buynow_LG.gif"
alt="PayPal - The safer, easier way to pay online">
</form>
On Click on buy now :
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<!-- Identify your business so that you can collect the payments. -->
<input type="hidden" name="business" value="herschelgomez#xyzzyu.com">
<!-- Specify a Buy Now button. -->
<input type="hidden" name="cmd" value="_xclick">
<!-- Specify details about the item that buyers will purchase. -->
<input type="hidden" name="item_name" value="Premium Umbrella">
<input type="hidden" name="amount" value="50.00">
<input type="hidden" name="currency_code" value="USD">
<!-- Prompt buyers to enter the quantities they want. -->
<input type="hidden" name="undefined_quantity" value="1">
<!-- Display the payment button. -->
<input type="image" name="submit" border="0"
src="https://www.paypalobjects.com/en_US/i/btn/btn_buynow_LG.gif"
alt="Buy Now">
<img alt="" border="0" width="1" height="1"
src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" >
</form>