Fix for compatibility issues with WooCommerce Wholesale Prices Premium

Hello there,

I’ve stumbled upon some compatibility issues with WooCommerce Wholesale Prices Premium plugin. Some of the cart discounts are applied as negative fees so Stripe payment cannot be processed and error “Invalid non-negative integer” occurs.

It’s pretty much the same issue as with “Conditional Discounts for WooCommerce by ORION” plugin so I fixed it with extending corresponding if statement.

Here’s a proposed fix that worked for me: https://pastebin.com/FNjqvpsn

Hope you can include it in a next release. ??