The Blockstream app operates on three Bitcoin-native networks: Bitcoin mainnet, Liquid, and Lightning. These networks have limited resources in terms of the volume or size of payments they can handle at any one time, so they charge fees as a way to handle the prioritization of transactions.
The fee amount and how it is calculated will depend on the network that you're using to send the transaction.
Note: Transaction fees in the Blockstream app are paid to those who process transactions on the Bitcoin, Liquid, and Lightning networks. These fees are not paid to Blockstream.
Bitcoin Transaction Fees
Bitcoin tends to have the highest fees of the three because it is the most decentralized and needs to keep the cost of operating a full node low. A new block of transactions is mined roughly every ten minutes on the Bitcoin network. Each block is limited by the data size of transactions rather than the bitcoin amount.
Due to the limited capacity of the Bitcoin network, fees can spike during times of high demand. In rare cases, fees have risen as high as $20 or more per transaction. Reasons for sudden spikes in demand can range, from fear of missing out (FOMO) during a bitcoin price run, to the deployment of new meta protocol tokens based on systems such as Ordinals or Runes.
Note: Due to the limited block space on the Bitcoin network and the unpredictability of the demand for block space, transaction fees in the Blockstream app are best viewed as estimates. In situations where the fee is not sufficient for timely processing, you can increase the fee to speed up the transaction.
Liquid Transaction Fees
Since Liquid is a federated sidechain for Bitcoin, its transaction fees tend to be lower than the Bitcoin mainnet. That being said, Liquid still has its own block size limit, and a functionary group generates a new block once per minute. If Liquid were to see increased demand for block space, fees could rise accordingly.
Much like Bitcoin itself, Liquid transaction fees are based on the data size of the transaction. Up to this point, there haven't been instances of demand spikes that have led to high fees on the network.
Lightning Transaction Fees
The Lightning Network generally has the lowest transaction fees (not including fees for opening/closing channels). Technically, Lightning transactions can have zero fees associated with them because the network does not operate as a blockchain. Instead, it is a secondary layer for payments built on top of Bitcoin and Liquid where transactions can be signed and cached before eventually achieving final settlement at the blockchain layer.
Notably, Lightning Network fees are based on the payment amount rather than Bitcoin and Liquid's data-size-based approach. If the fee for a Lightning payment is unusually high, it could be due to a lack of sufficient bitcoin liquidity in the Lightning nodes that are routing the payment.