I have a column of prices. All sale items end with ".99". I want
to return all items that are on sale.

I'm trying to use auto filter to find all items that end with "99"
or ".99".

In a helper column type

=IF(MOD(A1,1)=0.99,A1,"")

Drag down to extract all prices ending in .99

actually- it works for cells that are less than $10, like $9.99,
$1.99, etc. Anything with additional digits, such as $39.99, does not
work. I tried changing =.0.99
to"
=.99
=00.99
=*.99
="*.99"

There appears to be some rounding in the calculation.

Maybe something like this will help:
=IF(ABS(MOD(A1,1)-0.99)<0.0001,A1,"")