You need to send the handle-object of the payment term, not just the integer-based id. On a pragamatic level, you can actually specify directly as you have tried, you just need to envelope it correctly, as examplified here: https://api.e-conomic.com/secure/api1/EconomicWebService.asmx?op=Debtor_SetTermOfPayment
If it does not work initially, try to monitor what xml your application actually sends, and see if it matches structure in the above link.