This is normally done with a shopping cart. A multipage form, can be used as order form, but you need to know what and how to do it, it is not as simple. You would need to place different submit buttons (Checkout - Go on shopping) then in the next page have a script that will be able to see what the visitor has clicked and respond accordingly.

What you are trying to do here is a mini shopping cart. As such, it has some aspects that need hand coding by someone who can plan it and then code it. If you don't have this experience, you should either use one of the many shopping carts around, or have this custom script made by someone.

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