This is a top post - exactly what a forum should cover.
1. No reason why you can't shop in a supermarket or similar and add a few items on that you'd use anyway - food-related only I guess, otherwise your expenses' dept might challenge you. If you choose to eat 4 packs of crisps or drink 2 litres of orange juice, it's up to you
2. If you find a food shop, supermarket, etc where you can pay by credit card and get a non-itemised receipt, you can charge up to your full limit with no-one likely to contest your claim. M&S Food and Tesco used to do this but neither do now. In this case, it wouldn't matter if your items were food or not. If you get an itemised receipt too, destroy it before putting in your claim
3. Theoretically, you could probably claim from a credit card receipt, as above and then get a refund in cash or vouchers from the shop. Some places may insist on crediting your card, so not sure how that would pan out
Any other thoughts?