For this achievement the player has to deliver a total of 250 tons of cargo on 5 consecutive jobs with only Heavy Cargo Pack loads.
The following cargo qualifies for this achievement:
- Asphalt Miller - Writigen 808
- Concrete Beams
- Dozer Crawl - Z35K
- Industrial Cable Reel
- Locomotive - Vossloh G6
- Metal Centering
- Mobile Crane - Rex-Tex 45
- Transformer - PK900
If you go to the job select screen and choose a quick job, filter the selection on weight of the cargo, going from the highest to the lowest. The Heavy Cargo Pack dlc cargos are one of the highest and and will around the top.
You'll need to do 5 consecutive jobs, and total weight in total should be 250 or higher. I did two cargo deliveries of a Locomotive - Vossloh G6 (61 tons each) and three times Concrete Beams (51 tons each) waying a total of 275 tons, well over the threshold.
If you met the requirements, your achievement should unlock directly after delivering the 5th cargo.