Keep giftcard from applying to shipping amount, only on subtotal

Problem/Motivation

I'm running into errors, might be because of giftcards, but kind of wondering if it's doable to fix it prior to the error occurring.

When someone is using a giftcard and applying it to an order BUT the giftcard is for more than the SUBTOTAL and tries to apply the rest (but not all of the shipping). So for example, Product costs $14, shipping is $7, customer applies a giftcard with an amount of $15 and the total is showing giftcard is being applied to the $14 product PLUS the rest on shipping so the charge to Paypal should be $6.

I would actually like to be able to turn off the ability for someone to use a giftcard for shipping purposes too. Maybe separate from the issue that we're experiencing.

This WORKS when it covers the full amount of shipping, just not allowing us to process payments when the amount just barely covers shipping like the example above.

Steps to reproduce

1. Add item to cart
2. Go through checkout
3. Add a giftcard with a balance that would cover the item in the cart, but not fully cover shipping.
4. Attempt to click Pay with Paypal. Modal pops open to load Paypal

Proposed resolution

Allow a setting in the giftcard type to only apply giftcards to subtotal and not grand total with Shipping cost included.