if you look at the bottom of each receipt, there is already a unique identifying number.It's called the consecutive number. You can program it to be reset to zero at the same time as your daily figures, or you can program it to never reset.
in terms of the percentage keys

i wish i had a quid for each time i have had to go through this
Lets go through it for the benefit of all that follow..........
N1 - apply an amount or a percentage? you have already specified that you need a percentage so that option is a zero. Key is inactive? clearly you wish to use it so -no, the answer to that question is also a zero. Do you want the button to work only in X mode? - you didn't specify but im guessing that you don't - so that too is a zero.
zero + zero + zero is Zero! so N1 = 0
N2 - key is open or preset value - you want 3 preset values on 3 different keys so the answer to that is a zero. Apply to an item or to the entire sale - again, you stated sale so the value for that answer is Two.Dou you want people to be able to override the preset value of the button? from a security point of view, i would sanswer no to that - a zero.
so, zero + two + zero is Two so N2 = 2
N3 and N4 - is the rate negative or positive? You have already stated you are giving a discount so the answer is negative = Zero. Is the value to affect tax rate 1? You have not mentioned tax rates so i am assuming that there are no tax rates involved- this also applies to N4
The answer in each case is no so the value each time is zero - therefore N3 = 0 and N4 = 0
N5 - increase or reduce food stamp........I DON'T EVEN KNOW WHAT THAT IS!

I always answer no to that - so zero. Allow only 1 subtotal - yes = Two. [no = 0]
Allow multiple discounts without subtotal - not in your case so the answer is zero [if yes the answer would be four] so..... zero + two + zero = N5 = 2
N6 - Allow override in X - i wouls chose no = zero. compulsory validation - again i would chose no = zero -so zero + zero = N6 = 0
so now yoiu have the values for all the options, simply type them in in a string.- in your case ..
70 subtotal 020020 into % button followed by the same for each of the other two percentage buttons followed by the cash key to finalise.
Hope this helps you and puts the question to bed once and for all............[frankly i doubt it!]
Let me know how it turns out
Andy