Suspension upgrades

Seeking information on how to Beef Up my rear suspension.
And where to buy better Springs.
Heavy duty or towing.

I carry a lot of tools & pssangers sometimes. 300-700lbs sometimes

I assume for the 2006 Jetta sedan in your profile?
JSW towing springs will do fine if you keep 300lbs in your trunk at all times.

I have Tiguan springs in the front and JSW towing springs in the rear in my 2010 Jetta.
When I have 400-500lbs in passengers & cargo in the back half, it sits about level.

IDparts.com has spring sets and various options for struts/shocks.
I run Koni FSDs currently. A little stiff and jaring sometimes, but love the way it handles.
I gave my dad’s ‘06 Jetta TDI Bilstein TCs and Tiguan springs in the rear. Mild lift for larger loads - but with 300-700lbs you may find Tiguan rear springs insufficient.

Happy lifting.