Help with USPS First Class Mail Large Envelope shipping?

I'm thinking about selling some small items for the first time ever and I am thinking about selling stuff locally like to people in my town and a neighboring town that is about 20 miles away and I want to be able to mail stuff out at the post office with USPS First Class Mail Large Envelope shipping to people who want to buy stuff from me in the neighboring town-the items I'll be selling are less than 13 ounces in weight and I calculated the costs on the USPS site and the flat rate envelopes don't sound great to me because the town's just right there and I'd have to pay $5.15 it'd be a good option if it were like sending to 100 miles away. And the express shipping sounds worse. No matter which method I choose it says it'll be delivered the next business day. I want to use the USPS First Class Mail Large Envelope shipping method; it's priced at only $2.30 but I was wondering about the envelope requirements. I've seen some envelopes that look ideal for me like 4x6 inch ones or 6x10 inch ones but the minimum large envelope requirements say I have to meet the following size requirements: 6-1/8 inch x11-1/2 inchx and 1/4 inch thickness. I know I'll meet the thickness requirement but I'm confused on the rest; if I use a small 4x6 envelope what will happen?? And how will they base their price for shipping and will it cost less? And will I be able to do a delivery confirmation? And how long will it take to arrive to a place 20 miles away? Please help me out I am confused. I know all I need to do is buy a big envelope but I want to make my envelope look presentable to customers because the items will be very small like the size of 2 lighters(they aren't lighters though) and I only need a small envelope. Help me out I want to do delivery confirmation and get the lost price shipping that I can do but without sacrificing time waiting for my customers. Thanks in advance:)