Hello,
As you said PSTYP has a value table T163, can you please see what values it is taking as user-input at the table level in SE11? Is it taking the external format or the internal format? I have seen a similar kind of thing for PARWV (partner funct.). I think the conversion is automatic or else we need to use conversion-exits. The complexity increases when we are working with internal & external data formats for currency amount fields, quantity fields, UOM, ISO codes. You can refer to the links:
http://help.sap.com/saphelp_nw04/helpdata/en/a5/3ec9ea4ac011d1894e0000e829fbbd/content.htm
SAP Library - BAPI User Guide CA-BFA)
Please do some reading & that will help you clear the basic idea. I did the same to understand the underlying mechanisms.
Regards,
Tanmoy