95% of our customers do not need to read this help article. The only people who need to read this article are those with Platform Licenses (also known as Force.com Licenses) in their Salesforce Org. If this last sentence didn't mean anything to you, then chances are you don't need to read this article.
There are multiple methods to get Breadwinner to work for Platform Licenses. This article shows you how to clone and edit the Standard Platform Profile.
Clone the 'Standard Platform User' Profile
You cannot fully edit the Standard Platform User Profile, so the first thing you'll need to do is create a custom platform license. The easiest way to do that is to clone the Standard Platform User Profile.
Edit the Invoice object permissions and the Breadwinner Account Connection object permissions
Once you've created your custom profile, click on Object Settings, and then Breadwinner Account Connections. Click the Edit button on Breadwinner Account Connections, and click every checkbox on the page. Please note that some checkboxes cannot be checked, but aside from that, everything should be checked.
Once you've done that, Save, and go to the Invoice object, and click the Edit button. Check every checkbox for the Invoice object. Not every checkbox can be checked, but check everything that is possible.
Assign the Custom Platform Profile
Once you've created this Custom Platform Profile, go to the users with the Standard Platform Profile and change their profile to your new profile. With that, your Platform Users should be able to use Breadwinner! However, as the Salesforce Platform License does not include access to Opportunities, obviously they will have to create the Invoice from the Account record.
Permission Sets - Alternative Option
Instead of making a custom profile you can make a permission set and apply that to users. In that case, the instructions above will be roughly similar to what you need to do to create a permission set.