$5 is the service fee. It has nothing to do with the discount.

The discount is basic math.

$148 is the base fare

Now, subtract 10% for the web discount and get $133.20.

Take the discounted fare of $133.20 and add the $5 service charge to get $138.20 (which is off by a penny most likely due to rounding or SD in the math).

So, like the OP said, you can either book the base fare (of $153) at Travelocity -or- you can book the fare with the web discount (same fare basis code though) at $138.19, also at Travelocity.

Hope this helps!