I separate my envelope money into envelopes and then when I am at the checkout I estimate how much in in each category. If I am buying groceries and laundry detergent, then I’ll add approximately the cost of laundry detergent. I don’t worry too much if I’m off a bit.
If I go to someplace like Walmart or SuperTarget, I may buy things that are budgeted in several categories.. groceries, clothing, school supplies and household supplies. If I estimate that the clothing costs $20 and it actually costs $18 or $21, I’m not going to get picky over it as long as I am paying cash and am close.