There are so many elements that make up your invitation suite - it can be overwhelming! The above image will give you a starting point in explaining the details that go into the perfect suite. 
Sometimes, all elements aren't necessary - and at times, more are needed... yes, even more than what is shown. Different events call for different details, and remember, your invitation is the very beginning of the guest experience, and influences many factors including their attire for the evening, and yes, can even influence their gift to you.
So, how do you know what you need?
Ask yourself these questions...
1. What type of wedding am I having? Is it a formal affair that calls for all the bells and whistles, or a backyard BBQ which may only call for an invitation card + rsvp?
2. Is your ceremony and reception in the same location? If so, you may be able to get away without any enclosures explaining additional locations or directions.
3. Do you have a wedding website you'd like to boast about? A small enclosure is the perfect little reminder for your guests to check out your website.
4. Are you having a destination wedding? If so, you may need an enclosure explaining hotel accommodations or specific directions to your guests.
5... and MOST important... What is your budget? You may be having a formal affair and want all those beautiful features you've been pinning... but does your budget allow for this? Typically, your wedding stationery should take up approximately 5% of your total wedding budget, give or take. And remember to factor in postage for this as well... it's the little things we forget about that really add up!
(Don't forget... depending on the size or weight of your invitations, be prepared for an increased postage cost. It is a best practice to bring one invitation to the post office and have them weigh it prior to purchasing your postage).