I know the OP appears to be in Canada, however here in the US I recently found it's possible to buy 35% HP via Amazon, and I was able to get a quart of it delivered for about $25, which didn't seem terrible given the convenience and not having to deal with the weak stuff you get at a drug store. Apparently it's being used as a medication or something of the sort, as well as in water treatment. It's sold in smaller sizes but naturally the more you buy the less it costs, up to sellers that will ship out 6 gallons at once for around $180.
Using 3% is a headache and higher concentrations allow for faster reactions, plus you avoid dealing with large amounts of liquid that build up over time since you have to add so much 3% compared to higher strengths, especially when used to treat Cu-HCl.
Best wishes-
Larry