The network fee is what a blockchain charges to include a transaction, paid by the sender in the coin itself; it varies from fractions of a cent (Tron, Solana) to several dollars (Bitcoin, Ethereum at busy times) and is not set by the merchant.
Every deposit costs a fee to the network, on top of the amount received. On cheap chains it is negligible; on Bitcoin and Ethereum it moves with demand and can exceed a small top-up. The fee also applies on the way out, which is why very small balances cannot be refunded economically.
Choosing the chain for the size of the deposit is the whole trick: small and frequent on Tron, Solana or Litecoin, large and rare on anything.
At SMSMeteor: no deposit fee is charged by the platform; the network fee is yours, and the deposit screen shows the amount that must arrive for the credit you chose.