new liftgate struts are fairly inexpensive at rockauto.com. First clue I had any trouble with mine was overnighting somewhere very cold and when I hoisted / flung my hatch up and stepped inward under it, I got solidly hit on the head as it collapsed. New struts won't have any trouble with a trashbag added. As long as you aren't putting firewood in it.

eta
$20-50 for the pair, depending on your brand / perceived quality preferences
http://www.rockauto.com/en/catalog/f...t+support,1404