I will prescribe a cheap ,easy and fun DIY way.
1. Get yourself wood preservative (Preferably Copper naphthenate).
2. Saw off the tops at an angle.This helps in keeping water off the tops.
3. Armed with protective rubber gloves,goggles (don't put on tribal goggles please
), paint the lower 1/3 of the post and the angled top.Three coatings at intervals of about 2 hours will do.
4.On day two, coat the entire post with used oil and leave to dry.
5.You may spray the post holes with a pesticide on the day of propping.I would suggest you use Gladiator.One litre will take you very far and i doubt it costs more than 3K a litre.
6. DO NOT USE CONCRETE TO prop your POSTS!!!..Concrete retains water and sure,your posts will rot faster than where ordinary soil is used.
Besides saving, you'll agree this is a better way of spending your week-end than going to the par to kossip!!
Nothing great was ever achieved without enthusiasm.