Apps script: SumUp api requests giving error 400 - json

Question

I am integrating online payments to a web app. To do this I am using the SumUp API. It takes simple http requests. Here is the part of the documentation I am working with: https://developer.sumup.com/docs/single-payment/
My initial request for an access token from the API works fine. But issues arise when creating a checkout resource. I have checked many times and my JSON appears to be correct. The values all appear fine too. However when I run the code the SumUp server returns me this:
Request failed for https://api.sumup.com returned code 400. Truncated server response: {"error":"Unexpected token a in JSON at position 0"}
Here is my code:
var pay_headers = {
"Authorization": `Bearer ${access_token}`,
"Content-Type": "application/json"
};
var pay_details = {
"checkout_reference": "SH8Q0B5C", //random string of letters and numbers
"amount":10,
"currency":"GBP",
"pay_to_email": "docuser#sumup.com",
"description":"Sample one-time payment"
};
var pay_options = {
"method": "post",
"headers": pay_headers,
"payload": pay_details
};
var pay_response = UrlFetchApp.fetch("https://api.sumup.com/v0.1/checkouts",pay_options).getContentText();
Is there something wrong with this? I would appreciate any help as this has been a problematic issue. Thanks

Answer 1

The final 403 error here is due to a poorly documented requirement on the Sumup API. Basically you have to request that your account (whether test or live) is granted scope for the payments part of the API.
If you don't have this you can get an access token but will then be served a 403 error when creating a Checkout.
Save yourself some time and follow this: (I didn't)
https://github.com/sumup/sumup-ecom-php-sdk/issues/24
i.e. email integration#sumup.com
API docs:
https://developer.sumup.com/docs/authorization/#authorization-scopes

Answer 2

When I saw your provided document, the sample curl is as follows. Ref
curl -X POST \
https://api.sumup.com/v0.1/checkouts \
-H 'Authorization: Bearer 565e2d19cef68203170ddadb952141326d14e03f4ccbd46daa079c26c910a864' \
-H 'Content-Type: application/json' \
-d '{
"checkout_reference": "CO746453",
"amount": 10,
"currency": "EUR",
"pay_to_email": "docuser#sumup.com",
"description": "Sample one-time payment"
}'
If this curl command is converted to Google Apps Script, how about modifying your script as follows?
From:
"payload": pay_details
To:
"payload": JSON.stringify(pay_details)
Note:
When the above modification is reflected to your script, when an error occurs, can you provide the error message?
Added:
When above modification is reflected to your script, the request is the same with the sample curl command. From your following reply,
The thing is with the previous API request made in the script (the one that fetches the access token) the payload works fine without using stringify. I did try it for this payload however and it gave me a 403 forbidden. Here is the error message: Exception: Request failed for https://api.sumup.com returned code 403. Truncated server response: {"error_message":"request_not_allowed","error_code":"FORBIDDEN","status_code":403}. This error can be replicated by removing various other parts of the request too so I'm not sure whether it is beneficial to use stringify...
If your access token and your request body are valid values, from 403 of the status code, I'm worried that in your situation, the access from Google side might not be able to be done.
Reference:
403 Forbidden