What are the fees for a transaction?

Using LOBSTR does not add any additional fees for the end-user. Deposits and withdrawals are free.

The regular Stellar transaction fees still apply, however.

To prevent spam, each transaction on the Stellar network currently costs 0.00001 lumens when network activity is below capacity (standard operational state). 

When the number of operations submitted to a ledger exceeds network capacity (currently 1,000 ops/ledger), the network enters surge pricing mode, where a transaction may cost up to 0.001 lumens. Transactions that offer a higher fee per operation make it onto the ledger first.

This can result in what looks like an uneven balance in your account.

Read more on Stellar fees here: 

https://www.stellar.org/lumens/

https://www.stellar.org/developers/guides/concepts/fees.html