Hi, no we haven't done that yet. But by trying to search for information, I've found that the prepaid self-addressed envelope from Fedex can be a tricky thing.
This is the thread that seems to have some information.
With fedex:
1) it seems like you have to open a fedex account (also means provide your credit card #)
2) Buy a prepaid envelope but address it to yourself from the CIC. As if, CIC is the sender and you are the recipient
3) Also include your fedex account # that way if the envelope needs to clear customs and there are fees involved, CIC can charge your particular fedex account for such fees. But another post in the thread also explains how documents are valued under $20 and you wouldn't/shouldn't incur customs fees
With UPS, two different opinions on this:
1)
One post said they found it more helpful than Fedex,
2)
while another post said that the UPS store they went to couldn't generate a return label.
So this is a hit or miss depending on how knowledgeable the store employee is? Do your research and find out your options

Please keep us updated.