Xsolla's response to my request for assistance was as follows. "Our security system detected this activity as not typical for this account and instantly refunded the payment back to you. The alert might be due to you making multiple purchases in a short period of time, using several accounts or submitting a large payment amount for a newly created game account. Unfortunately, we cannot process your transaction at this time. We recommend that you try again with a debit or credit card. Thank you."
making multiple purchases in a short period of time - unavoidable as I can't purchase multiple at a time
using several accounts - unavoidable since they tell me they can't process a payment with one card, logical next step is try another card
submitting a large payment amount for a newly created game account - yes I purchased a package for a friend, so now I'm not permitted to complete my other purchases, that makes no sense.