I'd rather assumed that it must work that way, simply from the sheer number of pairs they'd otherwise have to stock. But it might be worth asking if you can keep them on for a bit.

If you do fancy a visit, weekdays are also feasible, as it's about five minutes from the office and thus easy for me to visit either in lunch hour or after work. I may use some of my flexi excess to skive off early and go to the shop.