Tanya, here's some young ones to compare to.

These first three pictured are all well under 100 grams and were captive hatched imported babies. They were not, at the time of these photo's, good eaters and some had to be assist fed to keep them alive. All three are now healthy snakes eating very well and gaining appropriately but it was a long road for them at first. However, they weren't as bad off as what you've described.







For comparison, here are three other babies. These three were good eaters, well started babies that were significantly bigger and hardier than the three pictured above.







Hope this helps to visually compare to that pet store baby. It sounds from your description that it is underweight and likely dehydrated. You may want to bring this to the store manager's attention with a caution that you will notify their corporate office and local humane law enforcement that they have for sale an obviously unfit animal.