I agree with PPs that it would not be rude to give the centrepieces to guests, but it may not be appreciated - perhaps particularly so with silk rather than fresh flowers, as since they are long lasting, people may feel that they are obligated to keep it / have it on show.
I would keep one or two for yourselves, and then let people know that they are welcome to take one at the end of the evening - that way, they will go with people who want them, there is no feeling of obligation, and if there is someone who you particularly want to be able to have one, and who you are *sure* will want one, you can always ask the venue to set aside one more than you want to keep for yourself, and then ask that person on the day whether hey wanted one, and were able to take one, and if the answers are yes and no respectively, you can give them your 'spare'.
My sister was married 2 years ago - they had origami flowers on the tables, and borrowed my collection of blue glass to hold them. I kept a few of the flowers but mainly because I liked how they looked in my blue bottles, and they were a nice reminder of the evening we spent before the wedding with the entire family sitting around the table making paper flowers.
If you want to give a special thank you to people who have travelled, i would suggest that you simply tell them so - in the speeches,but definitely in your thank you letters.
I have to admit that I would find it really odd to have a centrepiece mailed to me after the wedding - and potentially quite uncomfortable - it would feel like you have gone to so much trouble and expense that the centrepiece must be really imporant to you, which would then make be feel guilty if I didn't keep it and put it on display.