How to order the where clause and join columns in a covering index?
If all your filters are equality matches (such as =, rather than >, LIKE, !=, etc), then it's going to useful to list them first. Then the data that matches those three conditions will be easily locatable, and in CustomerID order, which gives good options for being used in a join. Remember that your matching on CustomerID needs to join on several things. 

Oh, and if your CustomerID is non-nullable and has a FK, then the Query Optimizer might not even be looking at t2.