This isn't SNAP, it's the basic income. Food, clothes, rent, fuel, school supplies, etc etc. It's pretty much every store.
Grocery stores handle it now by only selling SNAP eligible goods when the SNAP EBT is used. If you want anything else, it is rung up in a second transaction and you use a separate card. But your suggestion means they have to somehow signal to the system that I bought $28 in eligible food items, $15 in eligible school supplies, and $5 in ineligible items. But because there was only $27 worth of food items left on the card, it has to notify me that the actual total out of my other account will be $6. The back end would have to know what category each item is and charge it against a separate but linked account appropriate to that category, but as one complex transaction, so it will have to add a whole separate stage where it kicks back when one of the accounts is short and the general account is to be charged the difference.
And it's not just up to the merchants. The networks and clearing houses they connect to would all have to change as well, every single one of them. The changes would go all the way down to COBOL on dusty old decks written in the '70s. They're all designed on the principle that money is money and the charge is the charge.
I'm pushing because you're trying so hard to snatch defeat from the jaws of victory (hypothetically speaking).. And actually, you're the one pushing back, I'm fully on board with the basic income, you''re the one who isn't willing to go that far and wants to turn it into welfare 2.0 even if it costs an extra trillion. And if you don't think the tea party would use all those costs and all the hassles and all the confusion from perfectly responsible people confused and frustrated with all the extra complexity added to a simple transaction and use it as a bludgeon to get it dismantled, you are truly naive. (It would be hard enough to get anything like this done in the U.S. anyway).
I am willing to believe that the vast majority of adults in this country will spend the money in a reasonably responsible manner out of simple self preservation and love for their children. It would be an insult to those millions to treat them like children and it will do nothing to advance the goals of the program. I'm not sure why you expect otherwise.