As far as I know you can deduct the GST you pay from the GST you collect and remit the difference. You don't get to keep a %. As for PST it depends on what province you are in. In Manitoba....
If return is less than $3 you get to keep the entire return.
If the return is more than $3 but less then $20 you get to keep $3
If the return is more than $20 but less than $3000 the commision is as follows:
15% of the first $200
1% of the remainder
If the return is more than $3000 you don't get to keep any commision.
These rules for PST commision vary from province to province so check with your provincial tax office for more information.