1. Colour difference
There is a minor colour difference between the artwork you seen on the screen and the image printed on the thank you card.
Digital devices such as your phone or computer screens use RGB to display colours. RGB standards for red, green and blue. RGB Colours are not created with ink but with light.
All RGB files must be translated into CMYK for printing.
Certain RGB colours that you can see on your screens cannot be replicated with standard CMYK inks. If there are bright colours in your design, please check your image in CMYK colour mode to avoid potential RGB conversion issues.
2. Files needed for printing
Vector files such as .ai/.eps/.svg and PDF are preferred.
A vector image can be resize to any size without losing image quality.
Printing requires much finer detail, or higher resolution than a computer monitor does, and so the use of vector images wherever possible is critical for printing a clear and sharp image on the cardboard. If the image you provided is pixelated, the printed image will look even worse.