The domestic postage rates in effect in 1869 were:
3c per 1/2 ounce for transmission between post offices in the United States
Regulations (from 1852 PL&R) regarding Steamboat Mails:
Sec. 158. Masters or manager of all other [than contract] steamboats, are required by law, under penalty of thirty dollars, to deliver all letters brought by them, or within their care or power, addressed to, or destined for, the places at which they arrive, to the postmasters at such places: except letters relating to some part of the cargo. All letters not addressed to persons to whom the cargo, or any part of it, is consigned, are therefore to be delivered into the post office , to be charged with postage.
Sec. 8. The Postmaster General is authorized to provide and furnish to Postmasters, and other persons applying therefor, suitable letter envelopes with postage stamps thereon for prepayment of postage; and it is provided that letters enclosed in such envelopes with postage stamps thereon of a value equal to the postage which would be chargeable upon such letters and envelopes if the same were conveyed in the mails of the United States, may be sent, conveyed, and delivered otherwise than by post or mail.
Sec. 117. Persons desirous to send their letters by steamboats can most readily accomplish their object by enclosing such letters in the stamped envelopes issued by the department, inasmuch as letters so enclosed may be conveyed out of the mail without violation of law, and need not be delivered to the postmaster on arrival of the vessel.