Start here for reading and discussion and links: PureOS Optional Subscription Added to Advance Development
Choices of US$5.99, US$9.99 or US$19.99 a month.
You just order the level of support that you are willing to give in the Purism Store e.g. PureOS Subscription Premium – Purism
I am not aware that that has ever been explicitly answered but you can see in the linked discussion that it was asked. First raised in post #3 in that topic. Then posts #88 and #89. My guess is “no” but for enough €s everything is negotiable 
i.e. you can always ask.
As the linked discussion observes, if you are very concerned about exchange rate risk then you can make a one-off donation each time you want to and thereby have a much more accurate idea of the final amount in euros that you are donating each time.