[NickM]

No, my comment said calculate total purchases for the year as (reported income minus reported money added to savings) which specifically does not require tracking any purchases, only income and contributions to bank/investment/retirement accounts and such.

By not tracking purchases individually, and only inferring the total amount of money spent by subtracting savings from income, you can also apply brackets to purchases, and thus avoid the regressive nature of a "simple" flat sales tax.