Don't extend the deadline for her. My husband runs a business online with deadlines for payment every few weeks. Without fail, the same few people will pay late every time. They say things like, "I'll drop it in the mail tomorrow." Or, "Can I give it to you at church next week?" Those people are the same ones that he then has to chase down because they use the service and don't come to church the next week or "forget" to drop it in the mail.
Once you extend this for her, she'll expect it over and over.