You can spend $15-25 & have the bird DNA'd, but be sure you contact whichever lab you want to use, to be sure they do specie DNAs & can do them from freshly plucked feathers, then take the bird & sampling envelope/box to Petco, when the manager is there, pluck the sample feathers, place them in the shipping container, then both you & the manager initial and/or sign across the seal & send it off to the lab.....
Or
You could do the test on your own, get the results back & if you are right, file against the store in small claims court on the premise that they sold you a lesser valued bird so they could collect a higher price.....if you have a consumer advocate reporter with one of your local TV stations, you may want to get them involved.....national corporations do not like adverse publicity like this.....
Filing small claims is usually not expensive and the process is geared toward quick solutions rather than drawn out litigation...and...you don't have to know the law to purse your cause.....just be sure you've got all your ducks in a row.....
Heck, I could see them offering you $$$ rather than store credit.....even a bird of your choice, if that's what you wanted.....
Good luck
On reflection, I'd even contact the local newspaper, after talking with the TV reporter & see if they wanted to cover it also.....more press equals less happy the petco corporate offices will be.....