I will risk duplicating this tip as I can't remember whether I saw it here or elsewhere.
If you are buying Eurostar tickets, make certain to get to the British eurostar.com/uk site, not the US's eurostar.com/us site.
Reason? At the UK site the prices are shown in GBP, your transaction goes through in GBP, and your bank network will do a regular conversion. If you are in the US site, the prices are shown in USD, at their preselected Dynamic Conversion rate.
Example - today we purchased 2 tickets at 38 GBP each, and were charged by my bank $122.10, which is within cents of the expected conversion rate shown, On the US site, these tickets were $67 each ($134).
These 9% increases, while seemingly small, add up rapidly over time. Also note Eurostar's surcharge for using a credit card, does not apply to a debit card.