GST/HST registered sellers are entitled to Input Tax Credits (ITC) for all tax paid (GST or HST) on postage for shipments to Canadian buyers. The currency does not matter.
When doing your books, and tax return, you need to select a consistent method of accounting for currencies.
Some will use the actual amount paid in Cdn$ that day, some the exchange rate that day, other will average everything (revenues and expenses) on a monthly or an annual basis using the Bank of Canada exchange rate/ That is the method I have used personally for thirty years and I have been audited several times.
If unsure, discuss the matter with your accountant (tax preparer) and follow that advice.
Labels for destinations outside Canada do NOT include GST/HST and are not eligible for ITC. They are simply an expense like all other expenses (fees, packing material, etc...)
If paid in US$ yes you convert to Cdn$ and list them as expenses.
However, your earlier post reads "converted into CDN$ then paid to Canada Post" So if paid in Cdn$ all you need to do is add them up; no need to convert if already done and paid in Cdn$
