While SmugMug support staff check in here occasionally your best bet is to contact them directly.
SmugMug support explained that customers can pay through PayPal. A purchase test revealed that PayPal appears nowhere on the checkout page. Only credit card.
Does anyone use PayPal?
I found this (old) article saying that PayPal is an option at checkout:https://news.smugmug.com/new-checkout-with-paypal-on-print-sales-71d71f95ac65
Between that and support saying it is an option it seems like it should be there. If you don't see it I recommend going back to support to let them know it isn't there.
Thank you, all.