Should I ship my loud speakers to Canada, buyer says will pay all costs

This isn't an answer to your question, but you need to remove this from your returns policy:

I WILL NOT ACCEPT RETURNS OF ITEMS DAMAGED BY THE POST OFFICE.

If you read eBay's buyer protection policy you will see that eBay makes sellers responsible for refunding buyers for any loss or damage in the post.

Stating otherwise is not only meaningless but makes it more likely that buyers won't bother to contact you before opening a dispute through eBay. You need to avoid disputes as they damage your seller performance rating.

If something is lost or damaged in the post you have to refund the buyer, including postage, and then claim compensation if you can from Royal Mail.

As long as the buyer is satisfied with the item it's easy enough to send. Make sure you choose a delivery service that provides proof of delivery and has adequate insurance. Let your buyer know what service you will be using and reinvoice with the amended P&P price.

If it all goes pear shaped and the buyer turns out to be the buyer from hell you could be left out of pocket. Check their feedback carefully, particularly their feedback left for others.

You would also have to ensure that the shipping service provided coverage against damage to these particular items. Most courier companies have specific rules and exclude certain types of items.

It's also worth considering how you would pack them. I presume that they would have to be well protected and packed very securely to withstand a lot of 'handling' on the journey.

Two major matters you should be aware of and cover -

[1] Your buyer will have to pay customs charges and they could be quite high.

[2] UK to Canada can take a considerable time for heavy bulky items. Sending them signed for will make no difference if they are not willing to wait for delivery - once they open a not received dispute you will lose unless you prove delivery, so you need to send by a method to ensure the speakers arrive fairly quickly.