I don't work in finance, so for me the reason is "because finance won't pay otherwise", but I strongly suspect it's an anti-fraud accounting measure.
Would I pay more to be able to get invoices? No.
Well, technically yes, in the sense that I'm willing/able to pay $0 unless you can invoice, but otherwise, I won't pay an "invoicing fee" as a line-item surcharge any more than I'd pay a "printer paper fee" or an "electric lighting fee". Present the total invoice, and what you spend that money on internally is your business, not mine. If you need $1 for the invoicing process and I've agreed to pay $50, then $49 into one bucket and $1 goes into the invoicing bucket. I agreed to $50/mo, not $51/mo.
Would I pay more to be able to get invoices? No.
Well, technically yes, in the sense that I'm willing/able to pay $0 unless you can invoice, but otherwise, I won't pay an "invoicing fee" as a line-item surcharge any more than I'd pay a "printer paper fee" or an "electric lighting fee". Present the total invoice, and what you spend that money on internally is your business, not mine. If you need $1 for the invoicing process and I've agreed to pay $50, then $49 into one bucket and $1 goes into the invoicing bucket. I agreed to $50/mo, not $51/mo.