Most shops will and should charge you for doing this (fair price is about $40) since you are asking a trained employee to do it correctly for you. A good shop will ensure that the process is done correctly and if anything does go wrong they are therefore responsible for fixing/replacing the product.
Would you expect a professional car mechanic to install a part on your car that you bought somewhere else for free? Or a bike shop to do the same? No, of course not.
Your free option: do it at home, with the risk of damaging your liner and/or doing it incorrectly.
Not free option: paying someone a small free to ensure that it gets done professionally and properly.