Custom Printed Personalised 6 x 9 Small Polymailer Thank You Poly Mailers Envelope Postage Mailing S

I am interested in purchasing some mailers. What file type does it need to be? Do you send proofs? Do you keep the files in stock if I needed to re order? Do you offer other sizes than 6x9? What are the prices per piece and what are the shipping costs?
Item:Poly Mailers Envelope Postage Mailing Shipping Bags For Cosmetics
Feature:Eco-friendly/Tear-proof/Strong Adhesive/Perfect Printing
Size:10x13/12x15 inches/Customized.
Thickness:2.3mil/2.5mil/3mil/50-200microns/Customized
Durability:12 Kg or more
Industrial Use:Mailing, Shipping, Express, Clothing, E-commerce, etc.
Material:HDPE/LDPE/Co-Extruded Film/Biodegradable Material, etc.
Design/Printing:Custom Design Gravure Printing(10 Colors Max)
Material:Poly
Place of Origin:CN
Color:Pink
Color:Mixed Color
Color:Plum
Color:Yellow
Color:Chocolate
Color:Red
Color:Blue
Color:Orange
Color:White
Color:Black
Color:Green
Color:Custom Based on Pantone Color
Brand Name:Maibao Package