Rail install cost?

I’m going to look at some free-float rails. specifically this one in the photo. Is it customary or expected to pay for install? They are replacing an existing rail. I’ve pulled off all accessories and even the rail itself-just left with the barrel nut and of course the gas tube I didn’t remove either. On the phone they said, “come on down, you might have to wait a bit if we are busy but we’ll get you taken care of”..they really didn’t answer the question. So if I DO have to pay, how much is expected for a service like this?

I’m going to look at some free-float rails. specifically this one in the photo. Is it customary or expected to pay for install? They are replacing an existing rail. I’ve pulled off all accessories and even the rail itself-just left with the barrel nut and of course the gas tube I didn’t remove either. On the phone they said, “come on down, you might have to wait a bit if we are busy but we’ll get you taken care of”..they really didn’t answer the question. So if I DO have to pay, how much is expected for a service like this?