I think it's wonderful to feel like you have succeeded at a painting and would like to share it with friends and family, perhaps as a birthday or Xmas card or maybe you want to sell them! We are devoting two sessions to painting Xmas cards at the end of October/November but if you are looking for a greeting card printing company then where do you look online?
First of all you need to assess your computer skills, I have experienced many hours trying to use some of the card company online websites but have just become frustrated. The following points are things you need to be able to do to go down the online ordering, at the end of this I also give you a non online alternative if you are unable to do this.
Have a good copy of your painting - take a photo, a phone is good enough but make sure you have even lighting. Outdoors on a flat surface is usually enough. Make sure you get the painting in the frame without any distortion of the edges (i.e. the edges shouldn't be sloping in or out)
Second way of getting a good copy and slightly easier is using a printer scanner. Scan the painting at high resolution. Google says 'Place artwork face down on scanner. Scan the first quarter at a resolution of at least 300 dpi, preferably higher, taking great care to align the artwork squarely. Continue to scan all four quarters with exactly the same settings and again, take great care to align the artwork squarely."
Save your image for both of the above with a file name you will remember.
Once you have this then you are ready to try out the online printing for your cards. Here are three I have tried in the order of being easiest to use. They give you a choice of papers that your card will be loaded on to.
Things to remember - is your painting 'portrait' or 'landscape'? Bear in mind it may be tricky for the image to come out on a square card if you have a rectangle painting.
1. Moo Cards - a little expensive but easy you just upload your image https://www.moo.com/uk/greeting-cards
2. Printed - https://www.printed.com/products/greeting-cards - fairly easy more economical
3. Canva - if you want to design anything easy to use too, I haven't ordered cards from them but they do offer printing services - https://www.canva.com/templates/search/birthday-greeting-card/
For the Non-tech people:
Pop round to a local printing firm, check where they are online, most will scan the artwork and do some cards for you. Ring them up and explain what you need and whether they do it. They will ask you to take your painting in with you so they can scan it, leave the rest up to them! Ask for envelopes too. Most will have a minimum run though so depends on how many you need/want.
The one I have used in Worthing is PEP printers https://www.pep4print.co.uk/ they are very helpful and our Skyblue group has had Xmas cards printed there.