Does toner help color stay or cause fading?

I don't think it's going to really help make it last any longer. If your hair is still on the yellow side and you don't want it to fade to a corally color + want it to stay more pink as it fades, mix a tiny bit of purple or violet in with the Flamingo Pink, or make a separate toner with a violet so it stays more pink and less coral/salmon. Punky Color Flamingo Pink is a really good dye though, it should hold up well. You can do things to help keep it from fading as quickly though, like wearing a hat in the sun, keeping your hair up/out of the water, when you dye it cap it and warm it up with a dryer for 15 minutes and let it set about 4 hours, you can do a vinegar rinse after dying, use only cold water, sulfate free shampoo or CO wash, etc

You can also use a purple shampoo to help tone down the pink.  I used to do it when my hair was faded Cupcake, and it kept it a lovely cotton candy pink instead of a warmer salmon pink.