Coupon = Free (100% discount), shipping only generates PayPal Error

I have researched this on the internet (including MarketPress Support) and see many exact issues, some dating back a couple of years. I can't seem to find a final resolution for MarketPress including this thread from 3 weeks ago:

I set up a coupon at a discount of 100%. I'm using Table Rate Shipping. The objective is to offer the product for free but pay for shipping.

All is computed correctly
Product: $20.00
Discount: -$20.00
Shipping: $4.00
Total: $4.00

When I choose Pay by PayPal (using PayPal Express) I get the following message:

There was a problem connecting to PayPal to setup your purchase. Please try again.

10413 - Transaction refused because of an invalid argument. See additional error messages for details. - The totals of the cart item amounts do not match order amounts

Any guidance on this?
Merv (@lzw8bk)