8. Do I have to pay for postage to return my ballot by mail?

Yes. When returning your ballot by mail, you'll need to pay the required postage to send it to the US. Most countries do not provide free postage for ballot materials mailed from outside the US.

"US Postage-Paid" templates are available for download here for: #10 Envelopes [PDF] and C4 Envelopes [PDF]. 

But the "Postage-Paid" is limited to the US Postal Service. These templates are only effective when mailing your ballot back using the US Embassy's mail return service (via the diplomatic pouch), or US Military mail, or if you mail your ballot from within the US. 

You may also use private courier services, such as DHL or FedEx, to return your ballot. It can be expensive, but these companies may offer discounts for ballot materials.