eCart session variables
Help again please.
I have a <?php echo $_GET['pgx']; ?> from URL
The "pgx" refers to number of pages of a book.
I have a default number of pages with a fixed price, additional pages are charged for at a price per page. When I use <?php echo $_GET['pgx']; ?> to get total pages in the book my cart sees this as additional pages on top of the default pages, and calculates i.e. a 50 page book as 40pages default +(40 page default +10 additional pages) the calculations are for 90 pages not 50 pages.
How can I subtract the default number of pages (40) from the total book pages given
by supplied by the URL information <?php echo $_GET['pgx']; ?> so that the Cart Summary calculates the total number of pages correctly. I have tried a number of ways i.e. <?php echo $_GET['pgx']; ?>-40 but I do not know what I am doing. I have also tried in the eCart Object in the price calculations with no luck.
Is it easy, can you help