Does this item still work? I have it in my inventory, locked, to prevent it from getting stolen or distroyed...but it doesn't seem to be doing anything? Is it because I have the shop buff in effect rn that I got from RS?

I have the 20% card and that definitely works, and stacks with the shop buff too. You should be getting a message saying 'I see you have a discount card! I'll give you a discounted price of xxx' above the '(item) has been added to your inventory' message, and the shop buff discount is applied separately underneath in a green box. If you're not seeing the message about the discount card then it's not working and you should send a ticket/post in bug reports.

do you keep your's locked?

yes

Do the math and you will see.

SOLVED Please Lock!