You do not say how tall the stems are but, sometimes you can put a fence around to prop the stems. But if the stems have been bent "too much" and cannot stay straight up on a well watered plant, you may have to prune the worst offenders down to the ground and wait until the shrub develops new stems.

As for returning them, you may need to review the rules they have for returns in order to see if they will take them back.

Additional information would help.

Any photos possible?

Where are you?

Are they Limelight or Little Lime?

What are their planting conditions: soil, moisture/watering style and quantity, sunlight or shade amount? Is it windy? Do the leaves look limp?

If they are really Limelight rather than one of the other cultivars of H. paniculata, they should develop stiffer stems as they grow older, but the dense, heavy bloom heads will bend the stems downward somewhat. The way they are pruned (or not) can help develop stiffer stems or reduce the number of flowers so the branches aren't so weighed down. Many plants when newly planted may not grow entirely the way they will in the future in your garden. In the nursery they have been fertilized and handled differently than they will be growing in your garden, and they are currently still young (unless you got really large transplants, in which case you may be seeing transplant shock.)

Here is a blog entry from a Detroit area designer, Deborah Silver, who uses both Limelight and Little Lime extensively in her plantings, that show well-grown plants. Little Lime seems less stiff than Limelight. If you do a search for Limelight in her search box, she has photos of them in winter when she has pruned them along with pruning instructions.

You can also look through the thread in this forum on Limelight hedges, which has a bunch of photos with just a few photos that no longer show up, including one or two posters who have added a series of photos over several years as they grow.

You can also prune them into tree form but that would decrease the privacy they give and take some work to maintain in that form.

If you decide after seeing these photos that you don't like the look of Limelight when it's had time to mature a bit, look at Pinky Winky or Quickfire, both of which are large shrubs like Limelight, but have lacier, less heavy flower clusters and very stiff branches that in my garden don't bend at all, even under heavy flower loads.

Quickfire early July

Quickfire fall color

Pinky Winky mid-August just starting to change color from white to pink

Pinky Winky 9/21

You can see that the flowers stay upright and don't bend down on either of these, though the lowest branches of my untrimmed PW do emerge right at ground level.