Is it illegal (state or federal) to purchase a PSA eGift Card using a USPS money order mailed to one of their stores?

I called the Mt. Pleasant store, they would not accept a usps-mo through the mail to be placed on an eGift Card. The purchase of a eGift Card had to either be by using a credit card or coming into the store cash in hand. (I live in the middle of the county so the latter is out and I try to limit credit card use to an absolute minimum). Just seems odd and shouldn't be that big of a deal.