customer stretching shirt too too much with jpss transfer on it need advice

I am fairly new to making shirts and would like some advice on what to do here. I bought a pack of 50 sheets of JPSS. I have about 5 sheets left, and aside from the first couple that I used to practice with, It's safe to say I've sold at least 40 shirts with NO problems until now.

I made a transfer and put it on a Gerber onesie which I've found out recently that they shrink a LOT. So I quit using Gerber onesies and went with Faded Glory. Anyways, I sold a onesie using one of the last Gerber onesies I had, and a customer was unhappy with the quality once they washed the onesie. Here is a pic before I sent it to them...



Then here is the pic they sent me, unhappy with the quality after they washed it.



I assumed it was because of the onesie, since it was known to shrink alot. I had since started using Faded Glory onesies since they shrink less and feel more like a T-shirt. So I happily sent the customer a new one (told them to keep the first one) and they sent me an email today. They didn't even wash the onesie yet, but obviously stretched it out like crazy as soon as they got it, and this is the pic they sent me....



I have no idea what to do here. Is there another transfer that doesn't look like this when you stretch it? Am I doing something wrong? Or is this just what to expect from iron-on transfers and I should expect a customer like this about every 50 customers or so? I really appreciate any input here. I'm going to wait to see what you guys have to say before replying to them. Thanks!!