HST is to be charged by province. If a business is resident in Ontario, and sells to another province, the level of HST that is charged depends on the transaction. If a business in Ontario sells to a business in Alberta, and the Ontario business pays for shipping, the Alberta business is deemed to receive the goods in Alberta. Alberta HST (at 5%) applies. If, however, the Alberta company has a shipping contract and takes responsibility for picking up the goods in Ontario and shipping them, Ontario HST must be charged. In conclusion, HST charged largely depends upon where the recipient takes possession.
The above assumes the selling company only has an official presence in Ontario. If there is a business presence (i.e. an office or salesperson) or a deemed presence in a province, the transaction is deemed to occur within that province. For example if the Ontario company had a store in Alberta, the Alberta store would sell to Alberta customers at the 5% Alberta rate.