If your German visitors are well educated or technical then they will almost certainly have good English.
I wouldn't pain their ears with poor German; They'll think less of you. If you have good German then impress them and show it off otherwise work thru English.
I work for a German company which conducts all business through English.
I never studied latin, so Im definitely open to correction. I understood the cases were nominative, genitive, dative, accusative, ablative, vocative and locative
I did Latin at school and we declined nouns into five cases for each of singular and plural. The vocative and the locative pretty much overlap with other cases.
German has four cases and I found having learned Latin to be some use at the start. I find the German grammar tricky, probably as tricky as Latin, but the rules for putting sentences together are pretty much mechanical.
In the former East people do not necessarily have such good English, but most younger people (under 30s) have studied it at school.
bellum, bellum, bellum, belli, bello, bello, bella, bella, bella, belorum, bellis, bellis = the only thing I rmember from Latin.............the declention of Bellum (war) The first 6 are the nomative vocative genitive dativve and ablative of the singular form, and the last 6 of the plural form. Never learn the locative, though it's not that often you would want to address War in person (vocative!)
Where is the payment page on that site? All I can see are links to Barnes & Noble and Amazon.
By the way a payment page itself doesn't need to be secure. The URL through which payment details are submitted by the form on such a page should be though! However many sites make the payment page secure just to keep people happy.
I was looking at the Barnes & Noble link. Their page doesn't seem to be secure. I don't know enough about internet security to know if it's OK to give credit card details on it.