Posting as a large letter is cheaper than parcel post, but you cannot have tracking with a letter. There are size restrictions on letters, but at 7 x 7cm and just 0.7cm thick (including packaging?) this is OK. As long as total packaged thickness is less than 2cm, you can post as a letter.

The other cost consideration is weight: up to 125g costs $2, between 125 and 250g costs $3 and between 250 and 500g costs $5. If it is over 500g it cannot be posted as a letter.