A recipient of Rent Supplement can ask for alternative methods of payment - Postdraft (which you're getting now), EFT (via the bank) or Cheque. There may also be an option to pay the supplement directly to the landlord.
If the existing post office is awkward, is there another which might be handier for you - e.g. another town/village where you shop? Do you collect your weekly payment (pension, jobseeker's, etc) from a post office as well?
Let the Community Welfare Officer know about your difficulties with the Post Office location, and you may be able to come to another arrangement. If you are dissatisfied with the reply you receive, you could write or call to the Superintendent Community Welfare Officer for the area you live in.