What is the best way to mass send invoices via emai?

If your needs are simple enough, and you just want to send the invoices to the email address on the related customer, it can be done with a workflow. If you need to leverage complex logic, or want to send to multiple contacts, then a SuiteScript of some sort would be needed (multiple ways to do it). I tend to prefer Map/Reduce scripts for any task with the words “mass” or “bulk” in them, as that is what they do best.

I agree a workflow works well for this. Also as a side note if you setup the WF with the email action (or other actions you want) you can trigger it from a Mass Update so you can select which records to send or not send

is there a way to set the workflow subsidiary specific?

The best thing would be to add any filtering criteria to a saved search and used the saved search with the workflow. That way you can preview the populatin in the saved search results before the workflow is run

Yes there is a workflow condition section. However you can also make multiple send email actions with conditions for each subsidiary inside the same workflow

Are you available to help with this as a consultant?