Assume that a given coffee shop accepts bitcoin for payment where their policy is it is enough for them to provide service when they see a payment transaction in the mempool. Furthermore, assume that the average transaction fee for a transaction to be included in the next 3 blocks is $2.5.

If Eve is buying a $2 coffee, Explain how can she double spend the same coins such that the coffee shop's transaction doesn't get committed on the blockchain. Also, discuss her financial gain in this case. 

Avoid the trivial scenario where the initial coins, i.e., sent to the coffee shop, were already spent before Alice went to the shop.