Unfortunately, we are unable to tell you categorically how much it will cost for you to send invitations to each of your guests.
We recommend that you take a completed invitation set, with all pieces packed in your mailing envelope, to a post office. They will then be able to tell you how much it will cost to send each of your invitations.
Alternatively, for Australian customers, you can use Australia Post's online postage calculator to gain a rough idea of how much your invitations should cost to post.
Please note: As of January 2016, Australia's stamp prices are increasing to a minimum charge of $1 per envelope. You will need to ensure you're no longer using 70c stamps on their own, or you (or your guests) may be charged a fee for not paying the full postage rate.
Australia Post discrepancy: envelope size
According to Australia Post's online postage calculator, as of January 2016, mail is classed as a small letter (cheapest mailing) while it falls within the allocated dimensions (130mm x 240mm x 5mm, up to 250gm). Small letters now cost $1 to post.
Our invitation envelopes measure 130mm x 190mm, meaning they meet the size requirements of a small letter.
However, there have been some reports that some Australia Post stores/employees are stating that these envelopes are classed as "large letter", and therefore (according to them) should cost twice as much to post.
If you encounter this debate over the counter, we recommend that you direct their attention to Australia Post's online postage calculator. They should also have a cardboard mail slot which you can test the width of your envelope in. If they still insist that it's a large letter, despite all other evidence, you may want to try your luck with another Australia Post store.
We have sent many items in our standard invitation envelopes, and have never encountered this issue. However, we though it was worthwhile mentioning so you're not left unprepared if ever faced with this unfortunate, illogical argument!