Could the name on the sign actually refer to the making of multiple or plural signs at this business? Of course, it does.
So, this business sign should say "Signs."
What about the following sign? Is it correct?
You're right -- the sign is incorrect. It should read "Supplies" as that means more than one.
Here's a more difficult one:
Are multiple alterations offered in this business? Yes, so "alterations," is plural and correct.
Are the alterations meant for more than one lady? Yes. Then what is the plural of "lady"? Ah, if there is more than one lady, the plural is "ladies."
Are the alterations meant for more than one man? Yes, so the plural of "man" is "men."
If the alterations belong to all of the ladies and to all of the men who hire this business to alter their clothes, then we need to use an apostrophe after the formation of the plural to show possession (for the few irregular plurals that don't end in "s," add the apostrophe and an "s"). Therefore, the sign above should read, "Ladies' and Men's Alterations Available."
As you can see from the signs above, everyone needs to first determine if the word in question is plural or possessive. Then the answers will be much easier to "figure" out....unless, of course, your figure has changed proportions and you need to go to the business above for some grammar alterations.