One word for "happily surprised"

Consider thrilled (suddenly excited, given great pleasure), elated (made happy and excited; delighted; pleased), overjoyed (given great joy, delight or pleasure).

Although often used (and sometimes abused), amazed fulfils the need quite well. Its literal meaning is 'astonishment, wonder.'

I'd go with something like wonderment:

2: astonishment, surprise

Ken Dodd has the catchphrase “How tickled I am,” but has never been accused of using a formal register.

I would suggest gobsmacked.

Utterly astonished; astounded

Oxford Dictionaries Online

It enjoys more use in the U.K. than in the U.S., but may suit.