Padded gold colour mailers for 5- 6 cassettes inside their cases or 8 - 12 cassettes without cases,
F3 size which is the largest size for Royal Mail large letter size and pricing.
You can fit in 5 cassette tapes with their cases inside one of these envelopes and at a push fit 6 inside.
You can fit 8 cassette tapes inside the envelope side by side flat and possibly another 4 on top, however if you squeeze 12 inside it may exceed the Royal Mail thickness for large letter pricing, still fits inside no problem but charged for small parcel instead of large letter. However this may not be the case and you can still get it through as a large letter, depends on which jobs worth you get at the counter.
מק"ט: sealedairF3