Georgia - I was just taking a look at your shop. I'm fairly certain that posting a shipping upgrade as a separate listing is against Etsy TOU. You may want to fix this.
I don't know if you are able to list more than one shipping option yet, but if you are, that would be the most efficient way to allow buyers to upgrade shipping. The multiple shipping speeds is something you'll get if you join the Shipping Improvements Team here on Etsy. It may be available to others already, though, I'm not sure - I've had it for quite a while since joining that team though. I know I did read about someone else who was using it too.
For example, in my listings, I have my price for regular mail, then the options go up from there. Next up is the price for Expedited, and then Xpresspost. At this point, I just have the upgrades for North American customers in the listings - overseas buyers can request it via convo (I just have the regular mail price in the listing for overseas). It's worked soooo well for me so far - much less stress about having to wait on payments for upgraded shipping, etc., when the buyer can just select it in the check-out process.
No problems so far.
I wish other venues had this available too - definitely would have saved me the trouble I'm dealing with on ebay right now.
ps, I'm in Canada too, and I love your shop. Curious though - for your items, does it really cost twice as much to ship within your own country than to send to the US? Is it more expensive than overseas? I'm not criticizing, just curious... I ship small items, so I have never ever experienced domestic shipping being more expensive than international.. or are you automatically upgrading Canadian shipping?