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If you do not charge GST, you may want to investigate when you will need to start and whom you need to charge - check out [cra-arc.gc.ca...] if you are a sole proprietor.
GST only applies to transactions happened within Canada. Adsense with Google can be considered as providing (or exporting) services to Google Inc in USA. You don't charge Google GST (and you can't), so you don't pay GST.
The same theory applies to online store. When you sell a product to a customer from States, you don't charge GST. Thus you don't pay GST on that transaction.
Not sure what your question is with regard to the Canadian ad agency, but if you have billed them for services, you are liable for the GST. Even if you haven't added GST to your invoices, you must still work out the GST amount of your billings, set up an account, and pay the government.
This is of course apart from your personal income tax which is a totally separate matter.