Is this a postcard, or does a flap allow closure and sealing?

The OL423KW features four 4.25" x 5.5" postcards. There are perforations to separate the cards, but pre-scored folds are not included in this item. If you'd like a foldable card, we also offer the OL247KW - 8.5" x 5.5" Thank You Cards. This product provides two cards per sheet with a pre-scored fold. https://www.onlinelabels.com/products/ol247

Do you offer this cardstock in another color, neon or dark green?

At this time our cardstock is only available in white. However, we're always looking to add new items to our product line so we can forward your request for future consideration.

So I can make 2 greeting cards from one 8.5x11" by folding over half of it? Are the edges smooth after separation?

Our OL423KW features four 4.25" x 5.5" Greeting Cards / Postcards which can be separated at the perforations. If you'd like to have thank you cards, we also offer the following product which features fold lines and perforations to separate the sheet into two cards: https://www.onlinelabels.com/products/ol247 If you'd like to try samples of this product, you're welcome to visit the link below. https://www.onlinelabels.com/samples

So these cards support a full bleed(edge-to-edge) printing?

The OL423KW does not include margins so you'll need to check with your printer's printable area to see if it's able to print the edge. Please use the margin tool at the link below to find out what your printer’s non-printable margins are: https://www.onlinelabels.com/printermargins.htm

Does it has a preferation? I don't have the tool to cut the card stock.

Yes, the OL423KW is perforated so you can separate four 4.25" x 5.5" cards.

Can you print on both sides of the cardstock?

The White Cardstock (KW) material is the same on the front and back so both sides are printable. You would have to run the sheet through the printer twice if you don't have a printer that supports duplex printing. We do caution running the sheet through the printer multiple times or using the duplex printing option as it could potentially weaken the perforations or cause jamming. If you do experience any feeding issues, try using your printer's manual feed tray with a heavy or thick material type setting. You are also welcome to visit the link below to request free samples of this product for testing with your printer. https://www.onlinelabels.com/samples