How to pay U.S. passport fee without paying cash at a US. Consulate?

Why don't you ask the embassy how to overcome this problem?

It's complicated. You would have to apply through the consulate in Frankfurt. And it really is best to go there in person, which requires making an online appointment. Not sure if it possible to make a payment otherwise since they have senseless restrictions. My last couple of renewals I had to go to the consulate in Munich from Stuttgart. But even applying in person, one first needs to fill out the online application and take it with you. They won't print one out for you at the consulate according to their page!

And one needs to have the proper documents like the expired passport and passport photos which by the way have a different size and standards than in Germany. So the photo-booths one finds here where one can get photos for IDs and passports don't suffice. My last few passport photos I did on my computer and printed on heavy photo paper. The passports ae not issued immediately as they are all made in the States and sent over. But I know I recently waited a month for a registered letter to go from Germany to the States . And due to Corona, all expedited passports services are suspended. One can have the passport sent by post from the consulate rather than having to go again in person to pick it up but one needs to give them  a self-address stamped envelope

I am giving the link to the Frankfurt consulate page for US citizen services. Unfortunately it doesn't give a specific tel. number for passport inquiries like the Munich consulate page does but it mentions the email address: frankfurtü[email protected] to get such info. One should be aware that different US consulates in Germany have different rules.  One will strong forbid taking in a mobile-phone, even if it is turned off. Others don't even ask. I think acceptable forms of payment are standardized so if you can't find a number for the Frankfurt consulate then the US Embassy in Berlin tel. 030-8305-0 might be able to answer the question.

https://de.usembassy.gov/embassy-consul … -services/