Should I use Remote Functions or Remote Events for a shop?

Both RemoteEvents and Functions slow down the game if you fire/invoke them too much that it gets over its limit.

When you want to return values, use RemoteFunctions, if you don't want to return anything, use a RemoteEvent.

RemoteFunctions are pretty easy to use and are just like RemoteEvents:

-> Handled in server, invoked on client

-- server script

function RemoteFunction.OnServerInvoke(player)
    print(player.Name)
    return player.Name .. ' invoked the function'
end)

-- local script

print( RemoteFunction:InvokeServer() ) -- <player name here> invoked the function

-> Handled on client, invoked on server

--client
function RemoteFunction.OnClientInvoke()
    return game.Players.LocalPlayer:GetMouse().X
end

-- server

print( RemoteFunction:InvokeClient( some player ) ) -- some player's Mouse's x coordinate on their screen