I have a store that is also using the UC Coupon module. My customers can buy store credits that is applied to their account.
I only sell one product that is worth 1 credit. When the checkout and for some reason (they are customers) they would apply a coupon and they have credits left the Store Credit required would show a negative value. I would still work and 1 credit would subtract correctly from their account but would show up in the checkout as "Store credit: -0.999444 required, 1 available". Not very appealing.

I narrowed it down to line 33 in the JS file: uc_store_credit.js

var new_total = uc_store_credit_cart_total + line_item_total * uc_store_credit_line_item_rate;

If I add after that:

if(new_total < 1){
    new_total = 1;
}

it works.

Am I doing anything that could hurt functionality? It seems to me that it only reflects visually on the checkout form itself.