I tried Spree and Substruct and updated the latter for Rails 2.x.
But most of us actually just use: Gamer ActiveMerchant, if an existing Rails application already exists, and we just need to create a way for the client to sell the product, for example.
If you need your developer to create financial statements, there are several stones like:
fastcsv and ruby sheet spreadsheet for exporting data to excel / csv format basic reporting of sales, orders, etc. easy to Rails.
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