Although I am not a general fan of Oxycheq (having bought several Raider lights, and been through that nightmare) the Oxycheq analyzers work well, and if you are looking for a budget setup, the El Cheapo is great for DIYers.
Although almost every place from which we get fills has its own analyzer, there can be a problem if the tanks have just been mixed. I have picked up tanks where they have just been topped off with air, and the analysis in the shop is low; we have taken them home and analyzed them a day later, and the analysis is quite different. If you will always be picking up your tanks some time after they are filled, or if the shop always fills from banks or from a membrane system, that may not be an issue. But our shop does partial pressure fills, and adjusts the mix with air or O2 as necessary, and if you pick them up immediately after they are filled, the analyses can be quite off, but settle later to precisely what you want.