Using hitotsuzutsu when making purchases

I occasionally find myself in situations where I want to purchase one of everything (especially for small items like postcards, books, etc.). Easy enough to do when I pick out the items myself, but there was one time when I was at a bookshop and a clerk was helping me at the counter. I indicated that I wanted one example of every postcard on display, but when she saw I was having some difficulty thinking of the right Japanese term to use, yet somehow guessed what I wanted (I must have used some other construction that conveyed the meaning anyway), she stepped in and volunteered ひとつずつ?, then proceeded to pluck one postcard from each pile.

I just wanted to check with the community whether it's safe to use ひとつずつ in any situation of this kind, where I want one of everything - or whether it might mean something else in other contexts and I end up with a rather expensive mistake. Also, is it safe to just change the number if I want two or more of each (ふたつずつ, etc.)?

Thanks!

Yes, "hitotsu zutsu" means "one of each/every (kind)," so that would be safe to use. The same with "futatsu zutsu."

But I must admit that if it was postcards, the shop clerk made extra efforts to make it simpler for you. For counting sheets of paper, postcards, postage stamps, things that come in flat sheet shape," the proper counter is "-mai. So she could have said "ichi-mai zutsu" to you. But this "hitotsu, futatsu, mittsu..." being probably the first counter that non-Japanese learners of the Japanese language pick up, she kindly used the simplest, most common one I guess. :)