My client would like to sell gift certificates, in addition to having set prices they want the user to be able to input their own price. Is it possible to have a store item without a price?
As Ian has mentioned Store does not currently have the ability for users to enter their own price in. On exp-resso.com there is a channel for the donations (tip jar) that has a single product with a value of $1. When a user enters the price that they want to donate they are actually just adding X amount of items to the cart at $1 a piece so a $25 donation is actually registered as 25 donation items of $1 for a total value of $25.
I wrote a blog post a while back that deals with a similar situation.
As far as I know it's not possible to enter a product without a price as price, along with SKU, are required fields. One option would be to make use of modifiers to make any number of variations possible (£5, £10, £15, £20, ...).
There is also provision in Store to accept donations. This does allow for a custom amount to be entered and an example can be seen at the bottom of the Exp-resso website. I've not setup a donations form before however so I'm not sure on the specifics of setup or how it is recorded in the sytem (if it's in with the orders or stored elsewhere). I'm sure one of the Store guys can help to clarify though.
There is now a Enable Dynamic Prices option for the field that should work.
See see this post for some help.