@cooperlove: First of all, it is NOT true that every truly formal wedding has the words “black tie” in the additional information corner — the lower right — of the card. In fact, among people whose standards of formality out-formal the Duke of Cambridge, putting any such instruction on an invitation is unthinkably tasteless, as it suggests ignorance of decent dress standards on the part of their guests. Such people communicate the style of affair simply through the style of invitation: and engraved* black script on a white or off-white clean-cut heavy-paper card is communicating “formal” just as hard as it possibly can. But there are a couple of other details that go with this level of formality:
Is there an R.s.v.p. card? or (more formal and more proper) just the letters “R.s.v.p.” engraved in the lower left corner? or (most formal) nothing at all requesting a reply?
What is the exact wording of the invitation? Is it “Mr and Mrs Good Host / request the pleasure of the company of / Mr and Mrs Moku / to the wedding of their daughter / Mary Lucia / to / Mr Handsome Guy / on blah blah blah…? Or is it some innovation on the traditional form which would render the invitation less formal?
Finally, is the wedding in a church (most formal), or in a large home (almost equally formal), or in a rented venue (a little less formal)?
If your answer to all of the above is the “most formal” option, then I would generally recommend formal dress, except for one caveat. Since there is no mention of dancing, ball gowns and tail-coats would be over-the-top, but a long dinner-gown (no bare shoulders, no extravagant swishy skirt) and a dinner-jacket (tux) would generally be appropriate. Note that a dark suit (black, midnight blue, or dark charcoal grey with only a very subtle stripe if any) with white shirt can always be worn with propriety by a gentleman who does not own a dinner-jacket — but if you get many more invitations like this, you might want to invest in a dinner-jacket (and, of course, matching trousers) for your husband. Also note that, if this were a daytime wedding, that the correct dress would be a morning-coat with striped trousers and windsor tie for your husband, and a tea-dress with gloves, pearls and a hat for you. And that is where my one caveat comes in.
People committed to preserving the gracious formalities of social life, that is to say, those of us who care about the distinctions between white tie, black tie and between “dinner jackets” versus “tuxes”, also care a great deal about the appropriateness of dress to time of day. Morning clothes for visits and parties up until four in the afternoon or so (but six o’clock at the latest) and evening dress for parties from eight o’clock in the evening onward (but six o’clock at the earliest). And here is an otherwise-formal event starting at five o’clock that will certainly extend past six o’clock. What is a well-bred lady to do with such a conundrum? The answer is: cocktail dress. This is, in fact, exactly what cocktail dress was invented for. Those cute little “fascinators” used to be called “cocktail hats” — they function as a hat (no lady would go out in the afternoon without a hat!) until the clock strikes six — at which point they magically turn into hair ornaments (no lady would wear a hat in the evening!) Your cocktail dress makes a similar miraculous transformation. And your husband, in his dark suit with its silk tie and white shirt, is impeccable anyway, any time, because fashion and etiquette are much less demanding of the male of the species.
Do not ever carry a parcel to a formal event, unless it fits unobtrusively into your purse. Gifts should always be given discreetly and in private. Far better to send the gift to the bride’s home, regardless of the type of gift it is. Also remember that among people of old-fashioned formal manners, there is a rule that gifts may only be given by intimates, and must always be of value for something other than its costliness (a principle that can never be fulfilled by mere cash).
If you have been invited to a church wedding with no mention of the reception (which is an acceptable old-fashioned formal practice) then you are not considered an intimate and should not give a gift. If you have been invited to a home (or club, or hotel, or other rented venue) wedding, then the invitation implicitly includes both the ceremony and the reception, and you should send a gift.
*engraving is what text is called when it is raised on the front and dented in on the back — just like embossing but with ink on the raised portions. It is the only truly formal kind of commercial printing, and is the only commercia printing considered to be equal in formality and propriety to hand-written black india-ink script.