Hi, sorry this is confusing at the moment, we’re working on making this easier to understand.
Additional RAM is explained in our pricing docs. I can see from the invoice that you’re using only shared machines.
If, for example, you’re using shared-cpu-1x 512mb, then 256mb of RAM is “additional” to the amount given in the named shared-cpu-1x preset (which includes 256mb).
If you’re using shared-cpu-1x 1024mb, then 768mb is additional. This will cost 768/1024 * $5 (number of GB * roughly the cost per GB per month of additional RAM) = $3.75 per month per machine, if the machine is running for the whole month. Stopped machines do not incur any additional RAM charge (only rootfs charges).