Hi @nettspring,
This is a Geolocation + Tax situation.
You have your site set up so that product prices are inclusive of tax. This means that the €5 already includes your 19% tax for Germany and possibly most of Europe.
You’ve disabled shipping calculation on the Cart page
As such, any customer visiting your site from outside the region where you’ve set up the tax rate is not subject to taxes. This means that the tax portion of the price will be removed and they will pay the remainder.
If you take a look at this screenshot, you will see that the Net Price for a €5 that already includes 19% tax is €4.20:

Full Size: https://cld.wthms.co/eg1rcV
You can try out the tax calculator here – https://www.omnicalculator.com/finance/vat
When the customer is filling out their address on the checkout page, if the country of their address matches one of the countries for which you have enabled this tax, they will get charged the full amount. If their country does not match, they will be charged €4.20.
If you want the tax to be charged to all customers regardless of their location, then you’ll want to use a wildcard instead of specifying the countries that should get charged taxes.
You can learn more about setting up taxes in WooCommerce here – https://docs.woocommerce.com/document/setting-up-taxes-in-woocommerce/.
I’ll go ahead and mark this thread as resolved now.
If you have any further questions, do open a new thread.