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.