Poly bags must be transparent. The size of the warning text depends on the bag size. See “Poly bagged Units” at https://sellercentral.amazon.com/gp/help/200141500?ref=ag_200141500_cont_201100890&language=en_US

What I’d like to know is whether bubble bags (not mailers), which are made of transparent plastic layers, are transparent enough to be allowed if you put the bar code on the outside. Anyone know?

Amazon: Poly bags used to protect Units must meet the following requirements:

Poly bags with a 5" opening or larger (measured when flat) are required to have a suffocation warning, either printed on the bag itself, or attached as a label.
For example:
Warning: To avoid danger of suffocation, keep this plastic bag away from babies and children. Do not use this bag in cribs, beds, carriages or play pens. This bag is not a toy.
The warning should be printed or placed in a prominent location and in a legible font size for the size of the bag.
Print size of this warning should conform to the following table:
Total Length plus Width of Bag Minimum Print Size
60 inches or more 24 point
40 to 59 inches 18 point
30 to 39 inches 14 point
less than 29 inches 10 point
The thickness of the bag must be at least 1.5 mil (thousandth of an inch).
The poly bag must be transparent.
The poly bag must have a barcode (UPC, EAN, etc.) or X00-label that is scannable through the bag or have an X00- or ASIN-label on the outside of the bag.
Poly bags must be completely sealed.
Poly bag or shrink wrap must not protrude more than 3" past the dimensions of the product.
Note: Highly scented products (such as candles, incense, or bully sticks for dogs) must be sealed or bagged to prevent the absorption of the scent into other products.