Modifier - Discount

Hi all,

 I'am trying to set up a modifier that will provide

for example :

 

 - Customer AA has order £100,000 with in 3 months ( They can but any item but order amount all order summary £100,000) they will get discount £2,000(also we can choose percent or amount) or get 1 item YY free in same order or next order?

 - How to set up summary order amount during 3 months?

 

 Thanks in advance for help and guidance.

Best regards,

Prathana