Alternatively, you can go the MS route of purchasing GCs, converting them into MOs, depositing the MOs into a checking account and paying them with a check (assuming they refuse a CC payment).
I am in your boat in that many of my creditors do not take plastic. I have a special checking account just for them where the above is done.
I have calculated the cost of paying them in this manner is (without special sales or going through multiple payment portals, etc) to be 0.8%, which is a whole heck of lot less than the Plastic 2.5% or other "convenience fee offers" ranging from 2%-5%.extra over the base payment.