contract law

Alex runs a tour bus company for day trips. On January 2, nursing home manager Rose booked a day trip for the elderly. The basic terms of the contract set a date of February 3. Rose needs to guarantee a third COVID dose for all seniors. In addition, the contract stipulates that the bus needs to have air conditioning, leather reclining seats and first-class safety features. But on January 5th, Alex discovered that the air conditioner needed to be replaced. alex signs a contract with repairer A. The contract price is $3,000 and the contract date is due to be completed by January 13. Kayler, General Manager of Repairer A, informs Alex that the installation of the new air conditioner will be delayed. Kayler said replacement costs are billed per employee. But now an employee can't finish the job on time. Kayler suggested that Alex spend an extra $800 to hire additional staff to complete the task. Alex had to agree to pay an additional fee due to receiving an advance from Rose. Repairer A completed the replacement task on time, and the day trip to the nursing home was also held as scheduled. But Alex only paid Kayler $3,000. Kayler contacts Alex "as agreed" to pay an additional $800. But Alex only wanted to pay $3,000, and Kayler said legal action would be taken.

Discuss whether Alex is legally obligated to pay the additional $800 (IRAC) in the form of a resolution. Australian common law of contract. The rules and case law on sufficiency of consideration & practical benefit & promissory estoppel.