The only way I know to add eshop credit using a computer through Nintendo is by going through the process of purchasing a game via eshop credit on their website, which prompts you to add eshop credit before finalizing the purchase.
So, if you can find a game that's $20 more than your eshop balance, go through the process of purchasing it and then don't finalize the purchase. There could be an easier method, but this is what I've done in the past when I need oddball amounts of credit added to my account.
So if I'm understanding this right, hitting purchase right here on this Theatrhythm transaction page will only add the funds to my account, and not buy the game? https://i.imgur.com/Evdr61z.png
Otherwise I'm not too sure I understand this...
EDIT: Ended up just signing into my nintendo account on a friend's switch (connected to my nintendo network and paypal), adding the funds on there, then going back and buying the zelda season pass on my wii u while also having to add an additional $1.64 from my debit card. Kind of a pain in the ass just to save $5 but this is probably the easiest way if you know someone really close by who owns a switch.