Would my fellow jellies please help me prepare to attend a wedding?

It is less than two weeks away, and it is the wedding of the son of my fiancé‘s boss – who will likely run the company himself, one day. It’s a fairly small business and my fiancé has worked there for many years and is the head of his department. I have only met the owners and others once or twice with a smile and hello during brief visits to the office, so everyone is a stranger to me and I am quite introverted and uncomfortable / awkward in social situations. I have attended very few weddings, so I don’t know much about the current expectations.

First concern: “cocktail attire” is recommended – but weather-permitting, it may be outdoors at 3:00 in the afternoon in northwest NJ in late October. OK, no spike heels (which I can’t wear anyway), and probably something to layer on if it’s cool. What is “cocktail attire”? Suggestions for a full-figured redhead with arthritic feet?

Next concern: We ordered some items from their registry and had them shipped to their address (I’m glad for the convenience of the internet for that). Do we still give them a card on the day of the wedding?

I may have other questions come up as the discussion evolves. I thank you in advance for your help!