The au pair normally bears the travel costs himself/herself. You can thus be certain that the au pair will definitely come. Should the au pair change her mind in the last minute - which does happen sometimes, although quite rarely - you will not be disappointed that you had made an advance payment. In principle, we always recommend not to make any advance payments due to fraud attempts (scam).
On the other hand, we do encourage host families to partially or totally reimburse the travel costs to the au pair if they were satisfied with each other and both enjoyed the au pair placement. You can thus express your gratitude for the nice time spent together. This would also be a nice farewell present to your au pair, wouldn't it?