Service tax liability

Yes, you will have to pay service tax on that reimbursed amount also irrespective of the bill
 

If service provider has provided proper supporting bills aginst reimbursement bill then  no need to  pay  service tax  else service tax will be applicable on lumpsum reimbursement amount claim by service provider  .

follow the bill of TA, if they have included the bills which are seperately issued, i.e the payment is routed through TA, then service tax is to be levied on such charges, including their commission add on.