Minor editorial correction:
Okay, technically fish are a swing species in that there isn’t complete agreement, but there’s a pretty broad consensus.
I would say “group” rather than “species” here (since fish aren’t even a valid taxonomic group, let alone species.)
Since fish are vertebrates, I think the certainty is much higher than invertebrates in general. For invertebrates I am not convinced.
If invertebrates were conscious, it’d be very bad. For one, vertebrates typically lose consciousness very quickly when they sustain fatal wounds, due to blood loss which causes the brain to lose oxygen.
Most invertebrates’ nervous system is passively oxygenated. That means the brain keeps working long past the time when the rest of the body dies… even ex situ. It’s why invertebrates are used for studying neurophysiology, both because the neurons are so large compared to vertebrate neurons, and also because neural tissue continues to function after death.
If true, the wild animal suffering implications would be enormous.
I can’t speak to this specific case, but speaking generally, debit and/or credit card transactions can incur flat fee, making small donations not cost effective, especially if the card is in a foreign currency.
A sufficiently small donation can actually cost the organisation money, so it’s very common to set a minimum donation amount. $5/£5 is likely safely north of that amount, but because fees can be variable, you do want to leave some padding in.