The minimum balance for the active Stellar account is 1 XLM (was 20 XLM before Jan 2018), therefore when making a payment you should make sure to leave at least 1 lumen in your account.
This rule is forced by Stellar network (not Lobstr) and you can read more about it here: https://www.stellar.org/faq/#_Why_is_there_a_minimum_balance
You can reach to Stellar support with additional questions about minimum balance: https://www.stellar.org/contact/
LOBSTR already supports the new decreased minimum balance of 1 XLM of the Stellar network and you no longer need to have 20 XLM stored.
We understand that it could be inconvenient, as these funds are effectively locked in your wallet.
In case you really want to withdraw the last 1 XLM, LOBSTR provides you an option to withdraw all your balance without any restrictions to any Stellar address of your choice - learn more about our Merge Tool