"Can I somehow link my Canada Post account number to my PayPal account?"
No.
When you purchase postage through PayPal, you get a discount arranged by PayPal (and partially subsidized by eBay). The program is managed by Pitney-Bowes under contract from eBay.
If you have a volume contract with Canada Post, you will need to ascertain which deal is best for you: buying from PayPal or buying directly from Canada Post
Also, if saving money on postage is important to you, you may consider buying postage offered below face value by stamp dealers. In many instances the price will be much lower than what you pay at Canada Post or through PayPal - but - it does require some work matching denominations to the correct rates needed.
On $10,000 a year in postal purchases, you would save at least $1,000 - $1,500. Is it worth your time and effort?