Nice PayPal Code to Include cancel_url and return_url Statements

I am creating a WordPress localhost site to sell a product a member of my church produces and have installed the Nice PayPal plugin on the site. The person who created our church’s web site advised me that I need to include return_url and cancel_url lines in the code I’ve written, as in the following example:

cancel_url=”http://saint-albans.org/?page_id=687″ return_url=”http://saint-albans.org/?page_id=679″

The code I wrote, without the above lines, creates a drop down list of various quantities of the product and the prices associated with each quantity. It also creates a PayPal button which redirects to the seller’s PayPal page. The code I wrote is below.

[nicepaypal option1=”dropprice;quantity;Select desired quantity;1 gallon:30.00;2 gallons:50.00;3 gallons:65.00;4 gallons:80.00″]

I am not sure where to insert the return_url and cancel_url statements in this code. I have tried putting them at the end of the code and doing this removes the drop down and just leaves a PayPal button which redirects to the seller’s PayPal page but also forces the buyer to manually enter the dollar amount of their purchase, which I am trying to avoid. I’d welcome any help in getting this issue resolved.