Yes, Waterloo is a brand of sparkling water. Yes, Waterloo is a well-known ABBA song (Eurovision winner, 1974). But how is Tabby’s statement in panel 4 a punchline?
Edit: for those of you who wondered if Waterloo was a real brand:
I’ll Have a Napoleon Pastry With That
Comics I Don't Understand 
To a British person, Waterloo might sound like that drink they give you before the colonoscopy…
I think the humor in the last frame is just a sort of “anyway, as you were saying…”
ABBA famously wore sparkling costumes.
My take on Waterloo as a brand name: sounds too much like loo water.
I agree with ootenaboot. Is this a real brand? I mean really? What were they thinking?
Pete, (reload and) check the main post, where ZBicyclist has added verification images “for those of you who wondered if Waterloo was a real brand”.
Those of us who grew up in Waterloo (Ontario) are VERY aware of the brand, of course–kinda hard for me to miss!
That “deer in the headlights” look on Tabby’s face made me wonder if she had developed a sudden teenage crush on the customer (and was using the lyrics as a pickup line), but a comparison with other more recent strips revealed that she always looks like that: