Cannot checkout with coupons using Mollie payment since 1.9 (Mollie Orders API)

Describe the bug The new Mollie plugin introduced in Vendure 1.9 cannot create payment intents if a coupon is applied due to a VAT mismatch.

@martijnvdbrug

To Reproduce Steps to reproduce the behavior:

1. Use the Mollie 1.9 plugin
2. Apply a coupon code to the order, e.g. 50%
3. Attempt to create a Mollie payment intent using the corresponding mutation
4. Creation will fail as the Orders API expects a different VAT than what is supplied

Expected behavior Payment intent creation should work fine and applied coupons should be included the way Mollie expects them to: https://docs.mollie.com/orders/handling-discounts#discounts-and-vouchers

Environment (please complete the following information):

• @vendure/core version: 1.9 and up
• NodeJS 16
• Database (mysql/postgres etc): postgres

Additional context Example server output:

error 12/17/22, 11:45 PM - [Vendure Server] Order line 1 is invalid. VAT amount is off. Expected VAT amount to be €1.76 (€11.00 × (19.00% / 119.00%)), got €3.51
ApiError: Order line 1 is invalid. VAT amount is off. Expected VAT amount to be €1.76 (€11.00 × (19.00% / 119.00%)), got €3.51
    at Function.createFromResponse (/home/node/app/node_modules/@mollie/api-client/dist/mollie.cjs.js:355:12)
    at throwApiError (/home/node/app/node_modules/@mollie/api-client/dist/mollie.cjs.js:754:22)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)```

[michaelbromley]

@martijnvdbrug yeah, these rounding & tax issues are really horrible to deal with!

I think your solution is reasonable. Are there any implications to having a discrepancy in the unit price between Vendure and Mollie? E.g. when creating a refund for part of an order line (e.g. refunding 1 out of 3 of ProductA), does Mollie expect the unit price of a refunded line to be specified?

[martijnvdbrug]

@michaelbromley you can assign this to me