What goes wrong
Substituting a mailer for a Jiffy, then posting glass or a small hard item, is the damage.
Packaging Suppliers lists Jiffy envelopes for this line. A grey mailer is opaque film without that lining. It is cheaper and it is less protection.
The check before you reorder
A Jiffy envelope is the lined bag for an item that needs cushion and does not need a carton.
Store gold and white apart if you use both, and keep sizes in order.
Dispatch on that order
On Packaging Suppliers, put Jiffy envelopes on the order with the lines it belongs to, so you are not paying a second delivery for the piece you forgot. Free UK delivery starts over £50 ex VAT.