Invitation codes cost 21 XRP, yet you only have 0.999980 XRP after your account has been initialized. Why is that ?
Every XRP account has a 20 XRP reserve, which essentially holds 20 XRP as unusable. And, unfortunately, this XRP is un-recoverable. It is the cost of doing business on the XRP ledger.
Payburner also creates two micro transactions to register your public signing and encryption keys on the XRPL. For an explanation, click here.
21 - 20 - (2*.000001) = 0.999980.