Just because you have mailers doesn't mean you have to use them. Personally I'd sell the 25,000 mailers online, and allocate the cash to an area where you have expertise.
Your question is a near equivalent to saying: "I have a billboard in Tacoma, Washington. What's the best to market my business via that billboard?"
If you want to go the mailer route then start with 100 to 500 of them. Build a list of copywriting prospects. Send them a pitch with your best copy. Test different headlines, different sales copy, etc. Track what works and doesn't. Keep testing until you find a winning formula. Then mail the remainder of the 25,000.
Your question is a near equivalent to saying: "I have a billboard in Tacoma, Washington. What's the best to market my business via that billboard?"
If you want to go the mailer route then start with 100 to 500 of them. Build a list of copywriting prospects. Send them a pitch with your best copy. Test different headlines, different sales copy, etc. Track what works and doesn't. Keep testing until you find a winning formula. Then mail the remainder of the 25,000.