Whenever I need to remember something, I switch my phone wallpaper/lock screen to one of the default ones which I normally don't use. Then I can usually associate what I needed to remember. Works better than writing a note, or adding a reminder/alarm since these notifications is easily missed/ignored.

And then you forget where you put your car keys ...

I do this too. I always leave my keys and wallet in the same spot.

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.

Off topic but check if your bank has e-card for buying things online. The last 20 years I have only used temporary e-cards that are limited to one-time charge and a limited amount of money. No need to worry about your card details being stolen.

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.

I'd suggest memorizing your primary credit card number. You both don't have to remember to put your card back, and you don't have to find it before buying things online.