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I have found a range of gimmicked envelopes at Mark Leveridge magic. Anyone know anything about these? Anyone used them?

I have one of Mark's effects called ESP Image Maker (no longer available) which uses the Jumbo Leveridge Envelope. It's quite a clever gaff and made of durable card so should last a long while; well made, too (as well as you can make an envelope!). Can't be examined, though.

Without exposure can I get someone to drop one thing in it and later tip another similar thing out?

Yes, providing whatever you put in it is thin enough not to cause a noticeable bulge.