I would march over to that store with photographic and picture evidence that it is definitely not a pastel.
For example, I bet it doesn't have a clear belly. ALL pastels have a perfectly clear belly. You could use that as your base.
Refuse to take no for an answer and state that you'll put their name down the drain through the reptile community for scamming you.
Theres no such thing as a pastel x normal cross or a spider x normal cross. Sorry. :[ That would be a plain spider or a plain pastel, because both are heterozygous and have both the spider or pastel gene as well as the normal gene.
Take a scale with you if you have to and show that she is under the weight of which was claimed.