Keep in mind that feedback is irrelevant to your seller's account metrics and is in any case voluntary.
It is useful as a very rough guide to the buyer experience.
If your parcels are arriving (eventually) leaving feedback will make no real difference except emotionally to the seller.
If your parcels are not arriving, you have 30 days from the last estimated date of shipping to open an Item Not Received dispute on eBay.
If you miss that window you have 180 days from Payment to open an Item Not Received dispute on Paypal.
If the seller cannot prove delivery (not shipping, delivery) you will be refunded.
And the seller will get a Defect, which does have a drastic effect on his selling account.
Slow and no delivery are such common complaints that I prepared this boilerplate.
Not all of it will apply to you.
You have 30 days after that 'last estimated date for arrival' to open a Dispute with eBay's Resolution Centre, which is at the bottom of this page.
The first suggestion is to Contact the Seller. Don’t get into a conversation.
Just ask “When was this sent? What service was used? What is the tracking number?”
Don’t get into a conversation. You want either a prompt refund OR a tracking number that shows the purchase is in Canada.
Do NOT accept a replacement. It won’t arrive.
Do NOT close the Dispute until you have the product or the refund.
If you have already attempted to Contact the Seller, skip that step, ask eBay to step in and Escalate to a Claim.
If the seller cannot prove Delivery (not shipping, delivery) you will be refunded.
If the 30 days have passed, go to your Paypal account.
You have 180 days from payment for this Dispute.
Find the transaction and copy the number.
The PP Resolution Centre is at the top of your PP account page under Tools
Same process basically, skip the Contact Seller and escalate to a Claim.
If the seller cannot prove Delivery (not shipping, delivery) you will be refunded.
In future, read the feedback.
Don't buy from sellers with less than 98% positive (99% for Asian sellers)
Read the negative feedback , including seller responses, for patterns like slow or no delivery, poor communication, low quality, counterfeits.
If the seller left feedback for you on shipment, you can add a Response to that feedback indefinitely.