That's the kind of information you should lead with. You'd get more useful advice faster, that takes the specifics of your situation into account, instead of making people wonder why you're having so much trouble pressing buttons.
If you're on PC, you might be able to use JoyToKey to map controller buttons to keyboard keys, then set the game to keyboard input. I don't know if this works, but if it does work, it'd let you map whatever buttons to whatever inputs you want, so you could avoid having to press two buttons with the same thumb. You probably wouldn't be able to use the control sticks, though.
That means your card just can't be used. Try to purchase eShop on Amazon. I would say buy a $10 and $20 eShop card because of tax.
https://www.amazon.com/Nintendo-eShop-Gift-Card-Digital/dp/B01LYOCVZF