I love Gravity Forms. It’s the best form building tool for WordPress and now it offers a new feature that allows more complex calculations between fields. Combine that with the PayPal Payments Pro and you have a really fast, simple and powerful ecommerce option for those times when a full WordPress ecommerce system/plugin is not needed. Easier to set up, easier to maintain and gets the job done.
The sample below allows for a variety of shipping prices based on order total (but you could base it on total number of items too). It also adds sales tax – but only if your shipping address is set to California. Of course you could set it to any state. Note that the shipping field disappears when selecting a different state. The key is to set up a “subtotal” field and put your calculations in there.
There are some notes in this form – let me know if you have further questions about how this is set up in the comments below and I’ll do my best to help explain the options. See screnshots at the end of this post.
UPDATE 8/2013: I found this bit of free code from GravityWiz that gives you a ‘subtotal‘ field – I have not tried it but looks like a simpler solution that saves lots of work if you need to edit your form products with any regularity.