Yes, I will be happy to provide you with a certificate indicating that your document has been professionally edited. Most journals and funding agencies will accept this certificate as sufficient evidence that you have addressed any writing or language issues.
Ultimately, whether your manuscript or proposal is accepted will be based on a combination of factors, including readability, novelty and—of course—the science. However, if your edited manuscript or proposal is rejected due to poor language, I will be happy to edit your document again at no cost or refund your editing fee. Please ask for more information and conditions.