Calculated shipping costs on ebay only calculate parcel rates. You would have to set the lettermail rates yourself. Oversized lettermail is not printable on Paypal. You would have to purchase the stamps yourself from a Post Office.
Here are all of the lettermail rates:
http://www.canadapost.ca/cpo/mc/personal/ratesprices/postalprices.jsf
Just remember, in order to use lettermail postage, the item can not be thicker than 2cm.