Gannyrosie, If you have the poly mailers on hand use them. I use the plan brown 6 x 9 brown envelopes that I pick up from the dollar store 6 for $1 and they mail for slightly less than 80 cents. Other swappers send cards and pack the little squares in the cards. It is completely up to you. Everyone mails slightly differently.
It is so kind and thoughtful of you to ask about allergies. I have never asked nor been asked in the past. Let's see what every one says.