Can you write an effective sales letter without proof or testimonials?

I'm new to the copywriting business. I write copy for supermarkets, and I'm currently writing a sales letter to grocery store marketing directors to promote my copywriting services.

I've read a lot of literature on what makes for effective sales letter, and I think I have all of the key elements in my letter...except proof and testimonials.

Back before I knew what made a good sales letter, I had sent out a one-page "sales" letter to various retail and grocery stores. Somehow, this letter snagged me a steady client who has come back to me numerous times. She really likes my work, so I sent her an email a couple weeks ago to ask if she'd write me a testimonial at her convenience.

I got bupkis. Not even an email saying, "Sure, I'll get to it at some point."

The thing is, she's hired me for more work since I've sent that email. So I know she's read it. I don't want to keep asking her, though, because I don't want to be pesky.

So here's my Q: Since I can't get a testimonial, and most of the projects I've done for this client have yet to be implemented (which means no proof), can I still craft a sales letter that's going to get responses? I think what I have right now is strong, though not perfect.

Thanks!