Answer by Professional Japanese Teacher
You can use either expression to say “Welcome”. Both expressions are greetings to express feelings of welcome for visitors or customers at a store or restaurant.
Irasshai is the imperative form of the original word irassharu (an honorific term to mean “go”, “come”, or “to be”).
Both expressions can be used interchangeably to welcome customers.
However, Irasshaimase gives a trendy impression more suitable for the cities while Irasshai gives a friendly impression more suitable for bustling marketplaces and local stalls.
You may have to consider the positioning of your products and what image you want to portray at the Marché before deciding whether to go with Irasshaimase or Irasshai.
Sometimes, you can also switch between the two expressions depending on the customer. I hope everything will go well for you at your first Marché!