Solution: if the purchase amount is 30,000 yuan, the actual payment amount is 30,000× 0.9 = 27,000 yuan > 26 100 yuan.

So the actual amount of the second purchase is: 26100 ÷ 0.9 = 29,000 yuan.

The amount of two purchases is: 7800+29000=36800 yuan.

One-time purchase, payment: 30,000× 0.9+6,800× 0.8 = 32,440 yuan.

You can pay 7800+26100-32440 =1460 yuan less.

Answer: You can pay less 1460 yuan.