Joomla + VM3 - Amount in invoice

[davimari]

Hi guys, I just started to make my first store with VirtueMart 3 and I'm going crazy with a problem perhaps is trivial but I can't solve, and also on the net I don't find solutions

I need to add in the invoice body a voice with a fixed amount related to the total invoice amount

i.e. for invoice lower than 100$ an amount of 10$
      for invoice higher than 100$ an amount of 20$

Have had same problem? How you fixed it?

Thanks in advance for your grat help

David