I think they have charged you under the wrong code:
see on the attached:
https://www.police.qld.gov.au/rti/di.../RTI-15193.pdf
"13R(1)(a)" is the code you have been charged under, "Person in control of private light combination fails to ensure couplings comply"
i think it should have been "13Q(1)(a)" "Fail to ensure load on private light vehicle complies with requirements" - either way the charge is still $230
weather you should have been charged in the first place is debatable!
I would contest it, you shouldn't accept a charge that is incorrect, even if the fine is the same, what if you get stopped in the future when towing a trailer 'incorrectly' and they say you've already been charged for this offence, its not going to look good....
Tim

