Question:
Why is my multi-item form not working properly? It adds items I did not checkmark, or doesn't add items I do checkmark.

Answer:
Start with a fresh template from section 2 of Template.html in the ReadMe package:
http://www.dansie.net/cart_readme.html

Copy and paste an example form and test it without making any change to it except for the URL in the FORM tag. Then, make a small change, customizing it by describing one of your own items. Test it between small changes. If you do something that causes it to not work properly, you will be able to easily catch and fix it.

If you are using the Mall Version, don't forget the merchant tag.