This has just happened twice to me in a month.  The bride wants to order a small cake for show then getting or making sheet cakes on the side.  I know some people put a clause in their contracts that they cannot serve other cakes.

The first bride's mother was making the sheet cakes.  I thought at least their homemade and it's the mom so I allowed it and said I would make the cake.

Now another bride told me she wants a small cake and is serving sheet cakes on the side.  I asked where she was getting the sheet cakes, Walmart, vendor, etc.  She hasn't gotten back to me yet.

Has anyone else had this happen and what is your policy?

I don't do it if I know about it, but there's nothing that will prevent someone from bringing in sheet cakes without telling you. I have a policy that I won't do groom's cakes if I don't do the wedding cake, and I have a $500 minimum so people can't nickel and dime me with the sheet cake thing.