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.