If you'd like to contact the site owner/admin & get their approval to post, that's fine. I (personally) have no objections to your offer, but the forum rules you agreed to specifically state there is no advertising on the forum.
Offering a free service which includes fees for materials & shipping is in fact advertising, competing with the site's paid advertisers.
There are "contact us" links at the bottom of each page. You can use that link to contact admin and run it by them. If they say yes, great. Otherwise, the mods can not allow the post to remain.
"Some days you're the dog; some days you're the hydrant". author unknown