Your contract is with the seller, not with eBay, and your right to a refund in respect of goods which were not 'as described' (i.e. a
valid adoption package) is not time-limited (other than you'd be unable to pursue it through a court of law if you didn't take action to get your money back within 6 years).
Hassle the seller!
In the meantime consider paying £3 per month (or a lump sum of £36) to 'adopt' an animal for your niece directly with the WWF:
https://support.wwf.org.uk/
(If you go for the monthly option, an Extra £1.50 per month will get your niece into the Go Wild Club).