Tell us how often, if at all, you buy these items.
I searched what "if at all" means, but couldn't find the exact meaning that fits in this context. Does "if at all" mean "if applicable"?
If applicable is a good guess, but it's really closer to meaning if ever. The definition of "at all" is "in any way" or "to any extent."
How often do you buy these items,
if you ever buy them?
if you buy them at all?
"If at all" means nothing. It is a short form for:
Tell us how often you buy these items, if (you buy them) at all.
with the meaning:
Tell us how often you buy these items, if you ever buy them.