Add to cart HTML link

Hi I am having a little trouble with a HTML link.
I have a website for a charity orgonisation that raises money through a vehicle convoy.
I need the paricipants to fill out an online registration form (I'm using RSform Pro)
When they submit the form they are taken to the thank you page & when they click "Continue" I use the "Add to cart HTML link" to take the to checkout with the entry cost allready added to the cart.

My HTML code: www.mysite.com/index.php/checkout?option...ckout=1&product_id=7

The problem is that some people want to enter multiple vehicles and pay once, so I have a link back to a new registration page and the proccess starts over again.

When they click the continue button for the second time a second entry cost is not added to the cart and no matter how many times you fill out registration forms the cart never increases past 1 item.

I am out of ideas and desperately need help with this

Thanks to anyone who can assist

Anyone???

Thankyou so much, I know it was a simple fix but it was a case of "Can't see the forest for the trees"