[UPDATE]The “November Update” of 2011 has rectified this situation. You can now add only the amount necessary to make a single purchase. Way to go Nintendo. Very happy to be able to use it again. My wallet not so much.
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Nintendo’s Tech Support Forums: How do I get what I pay you for?.

Me bitching about the way the 3DS eShop works.

For example…

Since you do not offer per-instance billing, I buy $20 in “credit” in the eShop and am not charged tax on this purchase.

I buy a game at $4.99 and am now charged sales tax (including city sales tax even though the transaction takes place outside city limits, gee thanks) bringing the total to $5.44 leaving me a balance of $14.56. Repeat. Repeat. Repeat? No, I do not have enough “credit” left and will have to buy even more. Repeat.

Subtracting sales tax from a $20 purchase brings the actual total down to $18.18 available to use. What combination of purchases allows me to get $18.18 worth of software licenses? Is there some arcane formula hidden away that you can share with us so you aren’t just holding on to money that people have paid you that you haven’t delivered on?

Imagine if every store was run this way? Want to buy a $600 furniture set? Sorry, you’ll have to buy $1,000 of store “credit” first. Got $49.00 worth of groceries in your cart? Great! We offer $50 store “credit” vouchers. Ooo…. but with tax you’ll need to buy $100 of “credit”. We’ll just hold on to the balance until you either find a mysterious combination of purchases to zero the balance or have to buy more “credit”, whichever comes first.

What a rotten way to do business.

Why not do like the Wii shop? Round up the amounts to whole dollars and charge the tax upfront. Dilemma solved fairly easily. Or even better, offer per-purchase billing! It’s not hard, businesses do it all the time. I hear your number one portable gaming competitor does it millions of times a day no sweat.

Thanks in advance!

No doubt doomed for “moderation”.