I would definitely take a chance and try to clean them yourself. Here is
a link with some good tips.
It says:
"How do I get the rid of the yellow?
First, be certain that your shirt is washable. If it says dry clean only, believe the tag. Dry clean after each wearing to prevent yellowing. If your shirt is white and washable, here are two methods to try. Success will depend upon how old the stains are and the fabric content of your shirt – cotton whitens best.
* 1:1:1 – One part baking soda, one part peroxide, one part water. Make a solution of the three ingredients. You will need about 1/4 cup each of baking soda, peroxide and water to treat one shirt. Protect the countertop with a thick white towel, rub 1:1:1 solution into stains and allow to work for at least 30 minutes. Use an old soft toothbrush or bristle brush to loosen any residue and then wash as usual in warm or cold water.
* 1:1 – One part oxygen-based detergent (OxiClean), one part ammonia. In a well-ventilated room, wearing rubber gloves, mix solution well – about 2 tablespoons of each ingredient per shirt. Again, protecting countertop with thick white towel, rub solution into stains for at least 30 seconds. Wash as usual in warm or cold water.
For light colored shirts (blue, yellow, pink) test the solutions in an inconspicuous spot – perhaps along a hem – before trying the treatments under the arms. If there is no fading, proceed as for white shirts.
Darker colors may have stiffness or residue on the fabric under the arms. To remove the build-up and freshen the garments, fill the washing machine with cool water and add one cup of white vinegar. Allow shirts to soak for at least 30 minutes. Drain vinegar water and wash as usual in cool water.
Follow these directions and you’ll wonder where the yellow went when you’re dressing for your next event!"