Solidus Paypal checkout fails in sandbox for non-PayPal payment variants - ruby-on-rails

Question

I have built a webshop in Ruby on Rails using Solidus.
When I try to checkout in the PayPal sandbox with credit card or SEPA with a German address, it complains that the shop does not deliver there.
It works fine when I use PayPal payment with an account, also to a German address in Berlin...
This is the error in the ruby console:
Started GET "/solidus_paypal_commerce_platform/shipping_rates?order_id=R977809399&order_token=[FILTERED]&address%5Bcity%5D=Berlin&address%5Bstate%5D=&address%5Bcountry_code%5D=DE&address%5Bpostal_code%5D=10117" for 127.0.0.1 at 2022-09-29 20:29:49 +0200
Processing by SolidusPaypalCommercePlatform::ShippingRatesController#simulate_shipping_rates as */*
Parameters: {"order_id"=>"R977809399", "order_token"=>"[FILTERED]", "address"=>{"city"=>"Berlin", "state"=>"", "country_code"=>"DE", "postal_code"=>"10117"}}
Spree::User Load (0.1ms) SELECT "spree_users".* FROM "spree_users" WHERE "spree_users"."deleted_at" IS NULL AND "spree_users"."spree_api_key" = ? LIMIT ? [["spree_api_key", "undefined"], ["LIMIT", 1]]
Spree::Order Load (0.1ms) SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."number" = ? LIMIT ? [["number", "R977809399"], ["LIMIT", 1]]
Spree::User Load (0.1ms) SELECT "spree_users".* FROM "spree_users" WHERE "spree_users"."deleted_at" IS NULL AND "spree_users"."id" = ? LIMIT ? [["id", 3], ["LIMIT", 1]]
CACHE Spree::Order Load (0.0ms) SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."number" = ? LIMIT ? [["number", "R977809399"], ["LIMIT", 1]]
CACHE Spree::User Load (0.0ms) SELECT "spree_users".* FROM "spree_users" WHERE "spree_users"."deleted_at" IS NULL AND "spree_users"."id" = ? LIMIT ? [["id", 3], ["LIMIT", 1]]
TRANSACTION (0.0ms) begin transaction
Spree::Country Load (0.1ms) SELECT "spree_countries".* FROM "spree_countries" WHERE "spree_countries"."iso" = ? LIMIT ? [["iso", "DE"], ["LIMIT", 1]]
Spree::State Load (0.1ms) SELECT "spree_states".* FROM "spree_states" WHERE "spree_states"."country_id" = ? AND "spree_states"."abbr" = ? ORDER BY "spree_states"."name" ASC LIMIT ? [["country_id", 57], ["abbr", ""], ["LIMIT", 1]]
Spree::State Load (0.1ms) SELECT "spree_states".* FROM "spree_states" WHERE "spree_states"."country_id" = ? AND "spree_states"."name" = ? ORDER BY "spree_states"."name" ASC LIMIT ? [["country_id", 57], ["name", ""], ["LIMIT", 1]]
Spree::Store Load (0.0ms) SELECT "spree_stores".* FROM "spree_stores" WHERE "spree_stores"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]
Spree::Order Exists? (0.1ms) SELECT 1 AS one FROM "spree_orders" WHERE "spree_orders"."number" = ? AND "spree_orders"."id" != ? LIMIT ? [["number", "R977809399"], ["id", 10], ["LIMIT", 1]]
Spree::Adjustment Load (0.0ms) SELECT "spree_adjustments".* FROM "spree_adjustments" WHERE "spree_adjustments"."order_id" = ? AND "spree_adjustments"."source_type" = ? [["order_id", 10], ["source_type", "Spree::PromotionAction"]]
Spree::LineItem Load (0.0ms) SELECT "spree_line_items".* FROM "spree_line_items" WHERE "spree_line_items"."order_id" = ? ORDER BY "spree_line_items"."created_at" ASC, "spree_line_items"."id" ASC [["order_id", 10]]
Spree::Product Load (0.0ms) SELECT "spree_products".* FROM "spree_products" INNER JOIN "spree_variants" ON "spree_products"."id" = "spree_variants"."product_id" WHERE "spree_variants"."id" = ? LIMIT ? [["id", 45], ["LIMIT", 1]]
TRANSACTION (0.0ms) rollback transaction
Completed 422 Unprocessable Entity in 35ms (Views: 0.2ms | ActiveRecord: 0.8ms | Allocations: 31326)
Would be grateful for any pointers where to adjust that,
as I cannot make head or tail of it.

Answer 1

I don't know why Solidus is erroring internally, but I can tell you that if the rendered PayPal button has an onShippingChange callback yet that callback does not respond with success or times out (due to some error, generally), the PayPal checkout will give that message since you're indicating the address needs to be checked during payment approval yet are not responding that it's OK when queried.
Another possible flow design -- likely not used/offered by Solidus -- is to not specify onShippingChange and to check the address after approval (but before final order capture) and provide a mechanism to patch it when it's not a valid address you'll accept.