Is sending polly mailers for FBA small parcel delivery (SPD) fine?

Hi, I reached out to their support and read through all Amazon’s articles but can’t confirm if I can do this: I am sending five “boxes” via Small parcel delivery (SPD) to FBA (split shipments). However, I want to ensure that sending these units in a poly mailer instead of a box is ok. The products are clearly labeled and will be safe in the poly mailer. I am doing this because it makes the most sense; there is no box that small. I am sending 18 small units per bag.

If anyone has experience with this, please let me know 🙂

Thanks.