The 7.50 Q surcharge is a navigation surcharge that is charged by AC.

Here is a similar itin from ITA Matrix - you can see what the tax breakdown would be

Make sure to provide the exact booking and fare codes shown.
Fare 1: Carrier AC S21Q4TG ATL to YQB (rules)
Passenger type ADT, one-way fare, booking code S
Covers ATL-YYZ (Economy), YYZ-YUL (Economy), YUL-YQB (Economy)
$206.50
Fare 2: Carrier AC S21Q4TG YQB to ATL (rules)
Passenger type ADT, one-way fare, booking code S
Covers YQB-YYZ (Economy), YYZ-ATL (Economy)
$206.50
US Transportation Tax (US) $30.98
Toronto Airport Improvement Fee (SQ) $7.20
Canadian Harmonized Sales Tax (ON) (RC) $1.00
US September 11th Security Fee (AY) $5.60
US Passenger Facility Charge (XF) $4.50
US Flight Segment Tax (ZP) $20.00
USDA APHIS Fee (XA) $5.00
US Immigration Fee (XY) $7.00
US Customs Fee (YC) $5.50
Quebec Airport Improvement Fee (SQ) $26.30
Canadian Goods and Services Tax (XG) $1.30
Quebec Sales Tax (XQ) $2.60
Canadian Air Travelers Security Charge (CA) $11.50

Subtotal per passenger $541.48
Number of passengers x1

TOTAL AIRFARE & TAXES $541.48
This ticket is non-refundable.
Changes to this ticket will incur a penalty fee.
Fare construction (can be useful to travel agents)

ATL AC X/YTO AC X/YMQ AC YQB Q7.50 199.00S21Q4TG AC X/YTO AC ATL Q7.50 199.00S21Q4TG USD 413.00 END ZP ATL YYZ YUL YQB YYZ XT 5.50YC 7.00XY 5.00XA 30.98US 20.00ZP 5.60AY 11.50CA 1.00RC 33.50SQ 1.30XG 2.60XQ 4.50XF ATL4.50