Invoices in Gigwell send one at a time — there's no batch send. If what you actually want is for your buyer to make a single payment, the answer isn't sending several invoices together, it's building one itemized invoice that carries everything.
Build one itemized invoice instead
Check that every amount you want to combine sits on the same booking. This is the part that decides whether it's possible at all.
Open the booking and go to the Terms tab.
Under Artist Expenses, confirm every extra you want billed is entered as a billable expense with the right amount.
Go to the Payments tab and open Invoices.
Build a single invoice carrying the balance plus each billable expense as its own line, so your buyer can see what they're paying for.
Click Preview Draft, check the totals, then click Create Invoice in the Document Preview panel.
Click the invoice row and click Send.
If that doesn't work
If the amounts you want to combine sit on different bookings, they can't be merged onto one invoice — each booking bills separately. The exception is a run of shows for the same buyer, which can be held on one booking as a multi-date event. See the article on invoicing a run of shows.
Good to know
One itemized invoice means one payment, one reference and one thing to chase. It's usually the better outcome anyway.
Consolidating a balance together with expense invoices works the same way: build the single itemized invoice rather than trying to merge invoices you've already created.
If you've already created separate invoices and want to combine them, void the ones you don't need and issue a fresh itemized one. Invoice numbers are never reused, so you won't create a clash.