If we are going to play Four Yorkshiremen, then I'll tell you about the period of over six months when we couldn't afford to put the hot water on for a morning wash every day, never mind a bath. Stress about running out of money was something I grew up with whilst also autistic, which is why I have sympathy for the dysnumerate, the learning disabled, and the autistic who would be subjected to this needless added complication.
In a point of sale type scenario the person can see what they have left in each category as they put it in their basket they don't need to keep a tally.
Do you know what "point of sale" means in retail? It's the till. People aren't seeing how much money they have left in each category as they shop. They are finding out at the till when the computer says no.
A electronic scan-as-you-shop system restricts people to shopping in big supermarkets, not local small shops. Even Tesco Express doesn't have scan-as-you-shop, it's only the big branches that have that. The handheld scanners cost money. It's not worth a small shop's while getting them. Even Big Retail doesn't bother in smaller branches.
Green Shield and Co-op stamps weren't limited to one class of item, so much less complicated for the shopper than this voucher proposal would be.