I think I'm going to let my s/o pay for our groceries in total.
Sounds reasonable but why do you want her to pay for the groceries?
She lost a bet and I now want to collect my winnings
You shouldn't take advantage of your s/o like that. They may have lost a bet, but it's wrong to have her pay for all the groceries.
You don't understand me. The only way to get this money is to make her pay for some kind of physical item.
I understand where you are coming from, and honestly, I don't blame you because she should have paid you the money. I suppose I get it now. I guess it's easier for you to get the money by having her pay for the groceries than to get cash, so that's quite smart of you.