I'd do it with an additional card that just recorded the amount of every transaction up to (amount). Naturally, some would want to collect all sorts of extra private information. The key is to keep records down to the absolute basics, and preclude any meddling in what we choose to spend our money on.
Another idea worth discussing: Could we factor the extra road tax into the purchase price of a vehicle? That could bring economy and weight into play, with a slight trickledown in resale values.
There's a simple solution: mirror the current system and refund sales tax on the first (insert amount) of money spent, regardless of how much you make.
It's incredibly hard to break in without a citizenship in the EU. Though for the record, the guy who lit someone on fire with brake cleaner works with Team Lotus.