It is actually considered to be correct, in some circles anyway. I’d argue that it is one of these things that prescriptive rules don’t address adequately. There is an argument that apostrophes of this type can be used in plurals of letters and numbers, e.g. b’s, d’s and the 1960’s. However, this, I think, causes an element of confusion with genitive markers (denoting possession), e.g. John’s book, the dog’s bowl.

http://www.ucl.ac.uk/internet-grammar/nouns/genmark.htm

This is definitely not a contraction (compression as you called them) as when you expand the contraction it is grammatically unsound.

“There are literally hundreds is of(…)”

It appears it is a cardinal number version of hundreds which has been apostrophized as 100’s. It is correct. Though, personally, I’d prefer 100s, 1960s and so on. When no apostrophe marking is present in plural letters however, it does look a mess, e.g. bs, ds, as?

Apostrophes, not worth the hassle half the time.

Oh – and – regarding seed potatoes, don’t you have any small garden centres near you? Many of them will let you pick and mix from open boxes so you don’t have to be restricted to big nets which are probably more than you actually want of each type. Also likely to come from better sources than Homebase’s…