There are a lot of parts, that is true.

The invitation: Obviously you need this part, lol.

The envelopes: for a formal wedding, you would have an inner envelope with only the guests name on it, and then it would be put inside an outer envelope which would be addressed and stamped. The idea is so that the inner envelope doesn't get damaged in shipping. Most brides skip the inner envelope all together as most are just thrown away anyway and it makes for more garbage. Not very green.

Response Cards: If you need to know how many are actually attending, then you include these but there are other options. For example, you can ask them to RSVP by phone or email - and there are also online services that let your guests respond online instead of by mail. Again - much better for the environment to and it saves you money for the cards and the cost of the stamps so they can be sent back.

Reception Cards: These are used if you are only inviting some people to the reception. IF everyone is invited to both, you can include these details on the invitation itself instead.

Thank you cards: You need these but they don't have to be expensive. It is important to make sure each gift giver gets a personal thank you for their gift.

When to order: First, find out how long they will take to reach you, then make sure you give yourself a few weeks breathing room between arrival and the day they need to be mailed.