Before choosing an electricity retailer, what hidden fees should I look out for?

There are a few types of fees that you would need to know more about.
1. The standard 3rd party charges

2. Transmission Loss

3. non-standard fees

A1. The standard 3rd party charges are SP's transmission charges (Peak and offpeak charge), EMC, PSO, meter readings so on and so forth

A2. The result of transmission loss due to heat. Standard price plans are compulsory to absorb the transmission loss

A3. Non-standard price plans may be subjected to monthly fees, admin fees to name a few so do read the terms and conditions for every retailer.

Do look out for retailers that charging hidden fees like relocation fees, admin fees(there is a particular one but i wont name it, go do your research), etc. Look thru their factsheets. Hope that clarifies :)