Every time...
I have a theory for why OWIHTP happens: Shopping is exhausting. Having to process all the products in the store, minding the shopping list, and what you’ve picked up already and what you need to pick up what isn’t on the list, deciding between alternatives and sometimes reorganizing big chunks of the list because what you wanted to get isn’t available – is immensely draining.
When you’re at checkout and don’t have anything to do mentally, you give the brain a rest it gladly accepts.