actually I've thought of this last night too 
Took longer than I thought thanks to some complications 

Lazada Best Voucher Picker
Link: http://lazada.programmerjokes.com/

So just put the amount that you will buy and click submit then it will show the vouchercode with the best savings.
1st result is the one with the best savings

I am showing all of it sorted by the most saving voucher because:
1. Some are time use only and you've used it so you need the next alternative
2. Some are limited to certain category of goods(like food, electronics, n etc)
3. Some are for first time customers

Anyway, test it out and tell me if you found any mistakes in the calculations 

btw, I didn't show the amount after savings which is: 
your amount - savings = total payable amount