What is her style? Cakes, cookies, pastries? Do you want a single serving or a large assortment?
One place that comes to mind is Macaron Cafe, which is in the area. Would be fun to have plenty for herself and also share with the office.
More information would probably help you get more recommendations.
If she likes cupcakes I know that Butter Lane will deliver as well as Sugar Sweet Sunshine.
For Butter Lane, here's their web ordering form:
For SSS you can order direct through them or through MaxDelivery.com but it doesn't look like MaxDelivery goes to your sister's office location (29th and 5th = 10016 zip code).
Sugar Sweet Sunshine
126 Rivington St, New York, NY 10002
123 E 7th St, New York, NY 10009