If it’s like every other saddle and plate I’ve had, including fancy French one$, the saddle plate is held on to the back of the cantle by small nails/brads, there’s nothing complicated at all. NBD to remove it for engraving and install but you do need some kind of tool to remove it, like a flat screwdriver to pry it off . Way back when, I had a shoe repair guy remove one of mine for a few bucks, with a flat screwdriver to pry it up. Tack store used a small hammer to reinstall it after it was engraved at no charge. That was the first one, did that little chore myself on all the saddle plates since then.
Halter plates are more involved, usually some kind of flat screws with a flat backed…receiver?..don’t know what it’s called…on the inside where it’s against the horses face. Typically, any tack store that handles engraving and saddle repair will have the appropriate fasteners and tools or they (should) know who does.
If you have somebody around whose handy and has a few basic tools, they can usually handle these.