Some Advice for Brides & Bridesmaids
Once you sign that receipt, all alterations are at your expense. If you try the dress on before you sign and the dress is the wrong size, you can ask for a replacement. This does not include alterations to make a correctly sized dress fit your figure properly. This is for the cases where the dress is obviously the wrong size. You can't zip it up or you swim in it. And don't let them blame it on weight gain or wearing the wrong bra to stick you with the cost of alterations.
Chain-store bridal shops often make the bulk of their money through alterations. I think that's one of the reasons they can sell at such low prices.
Let the buyer beware. Try it on before you sign! Or just get a custom-made dress.