Be up front about it, Tell them that you are planning on buying the boot online before
you start trying boots on.
If I am not busy, I will most likely be happy to help. We all know what its like to be tight on cash. I am almost always happy to go grab a couple pairs for you to try on.
If I am busy, I will probably ask you to wait for a bit, or be much less involved.
Keep in mind even if I am flat out board to death before you came along, and you wait until after we have tried boots on to tell me you are going to buy online I am gonna be pretty pissed off.
So let the staff know before they start helping you.