All four volumes of this series are widely available secondhand, and the paperback editions would fit into a single package, so you could buy them here under the usual Fair Trade Book terms: $5 per book, $5 per package, $1 per online payment. (Yes, that's $10 or $11 for one book, $25 or $26 for the lot.) Out of this price I'd send $1 for one book or $4 for the set to Higgs or a charity of her choice.
If you can possibly afford it, though, I'd prefer to encourage you to buy the set as new books directly from Higgs' site. She put a lot of work into these books, they're still reasonably new, and they're a lovely long mellow read for those who enjoy curling up with a good clean novel. (Merry Christmas, Liz...)