I've had this problem recently, Jen. I personally think that, even though a 3100 gram female compared to a 2800 gram female is still breedable, you still should have got what you paid for and recieved what was advertised.
I recently bought an advertised "breedable," "1100gram" female pastel and got an 840gram snake in the mail. It is possible that she could have stuffed her with food for the photo in the advertisement, but it is false advertisement and bad business.
Jmartin's idea for advertising is excellent. I wish everyone did that.
However, I don't know if I'd let it slide, because I think a 600 gram female to an 800 gram female is a bigger deal than a 3100gram to a 2800gram. Returning it may be more trouble than it's worth, so I'd simply just take pictures of your snake in a scale and send them back and see what they have to say about the size difference. I would imagin that they all could deffinately eat a 200-300 gram meal, so that could explain a difference in weight, but I do not believe it should be advertised as such.
I think if you paid money out of your pocket for an animal, you better get that animal.
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Cheers!
• Chelsea Lynn Gardiner
(and Frank M. Wood)