Points.com handles mileage purchase, transfer, and gifting for several airlines. If you were charged excise tax this suggests that you purchased miles for a USA-based carrier, such as AA, AS, DL or UA, is that correct?

When you buy an airline ticket from a US based carrier the excise tax is included in the price, and normally not itemized. Assessing the tax at the time the miles are sold is the way the carriers have devised to collect the tax on award tickets.