From Muse end you can only integrate the shopping cart which your third party application provides , If you want to add Net 30 trade then you should include this in your shopping function which is managed from Ecwid.
Have you checked with them ? If not then please contact them for this purpose and let them know that you want to include Net 30 trade function with shopping feature , where users will be avail the discounts with bill pay period.
Thanks,
Sanjit