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100% of the invitations, except for maybe 5-10 will get tossed in the trash can - If you want your envelopes to look nice/professional, purchase a program like Print Shop or just use the envelope template for Word and print them out on a laser printer. Purchasing a low end laser printer is money better spent than putting money into creating fancy invitations/envelopes that are just going to be tossed in the trash.
IMO, they have some kick butt fantastic professional looking DIY invitation kits at Michael's and Office supply stores (Depot, Max, Staples, etc.)... I'd go that route because at the end of the day, you can set the tone without overspending on DIY invites that you could have just ordered a nice professional one already made for you that you only need to stuff the envelopes for, for half the price and none of the headache! The other advantage is that the companies provide a template for you without the headache of doing that portion yourself.
Also, regardless of which you choose - remember - postage adds up!!! The heavier the invite the more postage is going to run and this adds up SUPER quick!!!!
Another vote for don't waste your time & money on making fancy envelopes. Most people won't even notice & they will all end up in the trash. All it will do is take you more time to make them, more money on supplies & then also possible more to ship because it could had more weight to your invitation.