Change PID number?

Hello,

Is it possible to change the pid number in: https://mydomain.com/billing/cart.php?a=add&pid=2

I'd like to change this:
https://mydomain.com/billing/cart.php?a=add&pid=2

to this (for example):
https://mydomain.com/billing/cart.php?a=add&pid=1

Thanks :)