The English words “Custom number” were meant to refer to the ability of the customer to put his own custom, internal (identification) numbers inside a PayPal invoice. Without the whole context the translations came out as “Customer Number” and “Customs Number”. And strangely enough, “customary number” is the exact opposite of “custom number”… One has to get accustomed… 😉

In Danish:
Custom number  ==> Tilpasset nummer
Customer number ==> Kundenummer
Customs number ==> Toldnummer
Customary number ==> Sædvanlige nummer