" If a customer's purchases exceed 1,500,000, then the customer can return to buy more clothes later. If a customer's purchases are less than 1,500,000, then the customer will be credited with a cash voucher to purchase clothes at a later date."

This text is unclear. Because I don't understand exactly what you mean, I cannot correct it.

I don't understand what you are trying to say. Is invoice the receipt, the total sales value of what the customer wants to buy from you? What do you mean the customer has to make more money? Do you mean they need to buy more? But they already bought MORE than 1.5 (that's what exceeds means), so why would they need to increase the amount they are buying? And if they are buying less than 1.5, what is the spare cash voucher? You give them a voucher (maybe 1000) to use in your store on their next purchase? Why would they get a voucher for less than 1.5 but not more?