The materiality of letters

Here are the passages that caught my eye in this week's reading and made me think that we should have some blog discussion next week, in lieu of our canceled class, concerning the materiality of letters in eighteenth-century England:

Clarissa to Anna: “Mr Lovelace is so full of his contrivances and expedients that I think it may not be amiss to desire you to look carefully to the seals of my letters, as I shall to those of yours. If I find him base in this particular, I shall think him capable of any evil; and will fly him as my worst enemy” (529).

Clarissa to Anna, on Col. Morden’s letter: “This letter was enclosed (opened) in a blank cover. Scorn and detest me as they will, I wonder that one line was not sent with it—were it but to have more particularly pointed the design of it, in the same generous spirit that sent me the Spira—The sealing of the cover was with black wax. I hope there is new occasion in the family to give reason for black wax. But if there were, it would, to be sure, have been mentioned and laid at my door—perhaps too justly!”

Lovelace to Belford: "Dorcas, who is ever attentive to all her lady's motions, has given me some instances of her mistress's precautions. She wafers her letters, it seems, in two places; pricks the wafers; and then seals upon them. No doubt but those brought hither are taken the same care of. And she always examines the seals of the letters before she opens them" (570-71).

Another sentence that seemed to me to need glossing: "Subscription is formal between us. Besides, I am so totally hers, that I cannot say how much I am thine, or any other person's" (575).

What are the "seals"? What would "black wax" mean? (And what is "Spira," though this doesn't have anything to do with letters?) What are "wafers"? What is a "subscription," and what did letters usually look like? How much did they cost to send, and how were they usually sent? These are the sort of questions I'd like you to look into. Claim a particular aspect of the topic in the comments, if you like, so that everyone doesn't end up pursuing the same group--any useful links, facts or just thoughts on the matter will be relevant.