Refunds should be `is_payable`

Refunds should reduce the final amount the customer has paid.

Everytime you write `total_price(array('is_payable' => TRUE))`
you don't need to think/know about refunds.

Before completing a purchase a refund cannot be created so it
won't hurt the process at all.

After a completed purchase you often want to get the amount the
customer has paid. I couldn't find the case where you'd want
the original amount excluding refunds.

(The only case where you'd want the original amount of the transaction
is to compute the transaction fee, but this surely happens before
any refunds.)