Shipping callback compatibility with WC Name Your Price plugin

[Narek13]

PR Description

The problem is solved by adding compatibility layer for the WC Name Your Price plugin

Issue Description

When shipping callback is enabled, express payments for Name Your Price products may fail.

Steps to Reproduce

1. Set up a product with Name Your Price (don’t set any min/max prices)
2. Enable the shipping callback in PayPal Payments and enable the buttons e.g. on Classic Cart page
3. Add the NYP product to your cart with your own price
4. Visit the Cart page
5. Click the PayPal button
6. Confirm payment from the popup window
7. Observe buyer is redirected to order confirmation page
8. But there is an error stating that the payment provider declined the payment
9. User gets another chance to pay by clicking the Pay button
10. In the PCP log files, you will observe a patch attempt for a zero amo