Does anyone have any experience that they would like to share regarding higher level asks in a direct mail campaign?

We are considering testing a cold mailing to a targetted list of potentially high value givers. It would be an ask for a one-off gift of over £500.

Additionally, can anyone recommend a freelance copywriter/designer who has experience in cold acquisition for development charities? If they have experience of developing higher value asks, that would be even better.

Thanks