Your tag should beep when you travel through a toll point. If your tag doesn't beep, you'll be charged a fee of up to $0.75 per trip. To figure out how to avoid this, take the health check:
Check the installation guides here.
To order a holder:
Order a replacement tag
Transurban Linkt and Linkt are trade marks of Transurban Limited ABN 96 098 143 410