Cotton is a natural fibre, it is durable and will be reused. Natural cotton is biodegradable and compostable. It can loose it’s shape and crease long term but is probably the easiest and most effective material to print on. For a more sturdy rigid bag you can use a thicker cotton; often called canvas.
A natural UK stocked cotton or canvas bag with a front and reverse screen print will be a better environmental option than a bespoke size, 100% bespoke colour or gusset print, where the bags are imported for each individual order.