Usually, there is a remark field; if you have an uid say "xxx" in http://aktiv.ffii.de/ it would be appreciated to have that added in the remark field like " UID xxx" which greatly simplifies parsing. By adding the word "ISPUBLIC" into the remark field you can make your donation public, by adding "RECEIPT" or "NO RECEIPT" you can instruct us to deviate from default receipt behaviour.
If you donate more than or equal to EUR 50.01 and give a postal address in the remark fields and did not indicate otherwise, you will get a paper donation receipt usually within six weeks.
If you are transmitting from Germany, for a donation less than or equal to EUR 50.00 it might be smart to squeeze some extra info into the (electronic) transfer form, such as "SPENDE GMNNTZ KS VOLKSBLD" (abbreviating "gemeinnützige Körperschaft, Vereinszweck Volksbildung") or "MITGLBEITR GMNNTZ KS VOLKSBLD" that then also will occur on your account statement which saves us a receipt.
FYI: we do not have to pay any banking fees on incoming payments.