Hi, I'm having trouble in solving a business law question. Here is the case and requirement, please help:

"Billy is keen to start his own business selling frozen pizza on-line using the internet.

He decides to advertise his pizza by using the daily newspapers.In the advertisement he states as follows :-

High Quality Frozen Pizzas available for sale

We accept orders on-line at www.frozen-pizza.com.

Place your order now and receive a 50% discount off

the original price of $20.00 per packet.

Terms and conditions apply.

Mandy is Billy's friend. She places an order for 10 packets.She tells Billy that since they are friends she should get a discount. She tells him through SMS that if she does not hear from him within 7 days, she considers the discount as confirmed and agreed. Unfortunately, Billy is away in Malaysia and his handphone does not have an auto roaming facility.As a result, he does not receive the sms text message.

Amy is Billy's regular customer. She buys the pizza from him on a weekly basis and the orders are delivered directly to her door step every week.She pays Billy once every quarterly.Amy falls behind on payment to Billy for 2 quarters.When Billy demands payment, she tells him that since they do not have any signed contract between them, he does not have any legal rights against her for the outstanding payment.

Advise Billy, Mandy and Amy in solving their dispute

Thanks