Well girocards will give the shop the permission to make a one time withdrawal of the amount displayed on the screen, if that money is in your account. If the money isn’t in your account, the payment will not go through. Also if the shop wants to make a second withdrawal, you need to insert your card again and enter your (secret) pin again, they can’t choose what’s charged an when (only before you insert your card an pin, and only THIS transaction will be authorized).

Its a pretty secure system, as (as long as the card terminal isn’t hacked) you can’t spend more than you have, you don’t need to trust shops to only withdraw the agreed amount, and they can’t charge you a second time. Also the spending shows up on your account balance normally within 1 or 2 days.

From what I understand credit cards just let anyone make withdrawals of any amount, as soon as you know the numbers written on the card. So not only you need to trust the shop to withdraw only the correct amount, you need to keep track over you spendings really good, because they could just charge you an arbitrary amount of money on an arbitrary company name months after you gave them your details. Also from what I understand (normal) credit cards just will always work, and if you pay more than you have you just automatically accept a credit contract you need to pay back to your bank. Also (years ago, don’t know if still true) payments get charged to your bank account on bulk at the end of the month, which makes security and not spending to much even harder.