It is prohibited for sitters to ask for any compensation, but hosts can offer. I guess airfare wouldn’t be very economical, as one can probably get a sitter locally for a price. Or tempt a friend or family-member for a compensation.
Compensation is for many against the ethos of THS, so I believe many sitters actually wouldn’t like that such was advertised, but rather be a surprise or bonus on site if given.
For the holidays it seems sitters have plenty of options already. Not sure if a giftcard would sway sitters to sit if they haven’t planned to. I most certainly won’t. 