when you first use Phoenix to receive sats, you’re opening up a new payment channel, which does cost at least 3000 sats; I believe it scales up if you’re sending a considerable amount, although I can’t be quoted on how they calculate it

any subsequent sats received may or may not require new payment channels to be opened, as it depends on how much space you have in each channel and how few/many sats you receive

besides any typical routing fees, there is no charge to sending sats