If you're going to have self-service returns, just make everything free to take at that point
Nothing preventing you from getting a receipt from the parking lot, and returning the entire cart because "oh yeah I realized I didn't want anything from this 103,- purchase actually" and walk right out.
If companies are willing to lose this much to theft or refunds that would otherwise never be approved, just cut out all the in-between steps and make everything free, less headache to deal with as well. That's what they've basically done anyway, except now we also have to deal with tantrums when we refuse to refund their 5th purchase this month they apparently realized they didn't want anymore 0.002 seconds after buying it.