You need to review the contract you have signed with the agency to know exactly what your particular conditions are.
In many cases, the transfer fee only applies if you are taking a member of staff on permanently who has previously worked with you as supply staff through an agency. If you offer a post to a teacher through Airsupply, they are being employed by you.
Transfer fees should not apply if the teacher is still available to work with the agency. If you have employed the teacher through an agency in the last 14 weeks, you could potentially be charged a fee by an agency as a transfer fee. This is usually only a percentage of the teacher's salary amount for that role; e.g if you employ a teacher for a day @ £150 the agency may be entitled to charge you a % of this fee.