Formal:
* Avec amour de votre tante (With love from your aunt)
* Avec tout mon amour, votre tante (With all my love, your aunt)
Informal:
* Bisous de ta tante (Hugs from your aunt)
* Gros bisous de ta tante (Big hugs from your aunt)
* Avec beaucoup d'amour de ta tante (With lots of love from your aunt)
You can also use a more personal phrase, like:
* Ta tante t'embrasse (Your aunt kisses you)
* Je t'aime beaucoup, ma chérie/mon chéri (I love you very much, my darling)
The best choice will depend on who you're writing to and how close you are to them.