Exactly. As long as volume is right, and this is presuming you will brewing extract, your OG will be correct to the instructions give or take a couple of points, say +- .005 (this is within margins of error, IMO)
Call them and tell them you're missing it, maybe they will send it out next day delivery (call as early as you can, to maximize time to process) but worst case, it'll still be there before you need to test 2 - 3 weeks down the line when you bottle.