invalid zip code

You're not giving much information.

Germany is a different country to Ireland. The store may not deliver to Ireland.

Then ask them.

xxxxxxxx or 00000000 or 11111111 would be defaults

I presume you emailed them in German - if you emailed in English the response can be delayed

You mean enter it as it's usually shown?

When entering an Eircode, I've always been conflicted over the space between the digits. Ultimately, it doesn't seem to make a difference when posting within Ireland. But Germany!! I've had post/packages returned because I didn't include the two 'dots' that should be over the 'U' in Furth. I've even had post returned because I got the recipients surname wrong but the actual address was correct.

They seem to have a very unforgiving postal system.