If you decide to buy from another dealer, this essential information is still yours to use. Charges vary for this service, and many designers will refund this charge if you buy the gear from them and have them install it. It is common for installing dealers to ask you to pay for some or all of the hardware prior to installation. This allows them to use your capital to finance the job. It is not uncommon for installing dealers not to have all the equipment for your system in stock.
Inventory costs money.
You should never have to pay for installation labour costs until the system is installed and working to your satisfaction. It is not uncommon for installing dealers to refuse to install hardware they did not sell. Installing dealers are working on very slim margins, as they are beset on all sides by competition from companies that offer low prices instead of quality, onsite service. At this point, money changes hands. Everything must be on paper, one copy for the installing dealer and the other for the system customer.