Hi Owen
USPS First Class International usually takes 7-8 days to arrive. I've had single cards arrive with 98c on the postage receipt, most are around $1.70 though. I wouldn't pay any more than $5 but if it's valuable you may want to pay for extra in terms of insurance, express or registered mail.
it's $1.71 the correct postage anything else above that you may have to consider your paying for the toploader/penny sleeve and their holiday to Europe , can take up to 21 even 30 days, the quickest i have ever had was about a week the average is about 2 weeks
As a US Seller, I usually see my bubble mailers with a single card and toploader and team bag go for around $3.50 with a customs declaration. The price tag the Post Office usually puts on it is $2.54 to Australia and most other international countries