hello,

while calling on change check whether partner is customer then only execute your code. As customer and supplier form, Both shares same object

You can eitehr add customer field to your onchange and check whether you are getting that field and accordingly execute or update context by passing customer field and check on your onchange method.

Here change for your xml , on_change="onchange_zipcode(zip, customer)

and in onchange method,

    def onchange_zipcode(self, cr, uid, ids, zip, customer, context=None):
        res = {}
        if not customer:
            return res
        #Execute your code, This code will be executed only if customer is set to true

Hope this will help you.