How to make a billing system using visual studio 2017

//OUTPUT 1
COMBO A,B,C,D  / a,b,c,d ( e = exit) :a
Quantity : 2
    COMBO A: 2 X RM10 - 6%= RM 18.80
COMBO A,B,C,D  / a,b,c,d ( e = exit) :e
            Member discount =      6%
            Combo Charges = RM 18.80
            Add  SST      = RM 2.00
            TOTAL TO PAY  = RM 20.80
            Amount Paid   = RM

//OUTPUT 2
COMBO A,B,C,D  / a,b,c,d ( e = exit) :a
Quantity : 2
    COMBO A: 2 X RM10 - 6%= RM 18.80
COMBO A,B,C,D  / a,b,c,d ( e = exit) :1
gg
COMBO A,B,C,D  / a,b,c,d ( e = exit) :e
            Member discount =      6%
            Combo Charges = RM 37.60
            Add  SST      = RM 4.00
            TOTAL TO PAY  = RM 41.60
            Amount Paid   = RM

Hi, I really need help on this, as you can see,my output 1 and 2 have the same quantity of COMBO A, so both the price of combo charges should be RM18.80, but it came out different prize on combo charges after i press integer on the 'COMBO A,B,C,D' on the OUTPUT 2. So can anyone help me solve this problems, tell me whats my problems and what should i type?