I import daily (invoiced) sales from a Prestashop server as journal entries. It's not an ideal solution in my case, as I'd really like to keep track of payments/allocations using FA, but for POS data it's all you need.
Since you basically only need to split the total between sales and tax, and book the amounts to the appropriate accounts, you could use a "quick entry". And if you manage to get the data out of Unicenta programmatically, you can feed it into FA with a single curl call. (Actually two. You'll need one to login and create a session.) You wouldn't even need the CSV import plugin I use.
Let me know if you need any help.