Whenever I need to remember to do a one-off thing in the morning, I just leave my car keys next to (or under) something that will remind me. That way I literally can't leave without remembering to do it.
It majorly screws up my routine if I pick up my wallet to pay for something online and forget to put it back where it belongs, and I can't find it the next day. Or I leave it in the office and the next morning I have to remember I left it in the office, and it's not hiding somewhere at home.
Some places like paypal might be hard but I don't use paypal anyway because of all the problems they have caused the people I bought things from. If my e-card is not accepted, I can probably do without the item anyway.