Hi Debbydoccious --
I do not think it'll do much damage (not a lot of calories), and as it is protein it should not slow your getting in to ketosis by much.
However, if oyu are determined to do SS and not SS+ or and 810 plan... then someone on this forum suggested having an extra pack if you are struggling.
This means that you might run out early if can't get more from your CDC, but Tescos makes a diet meal replacement that might do in a pinch, as do many other companies (Boots, etc. have things).
(I have extra CD packets for just such a thing: very hungry or difficult day... I bought extras and said that I liked variety and might chang emy mind on what I felt like having-- which is true.)
MinnieMel