It's all in the personal touches that really makes you stand out from the rest. From the Save the Dates, Invitations, On the Day Stationery to the Thank You cards, I think it's important to keep consistency throughout and make everything unique to you; your guests will appreciate it and remember it too, which is what you want.
Whether you want your photos from the wedding day on your cards, or you want them to be in keeping with your invitation theme, having them designed professionally will make such a huge difference compared to having them done online for a number of reasons; 1. You get to see a digital proof that we can amend to make sure they are absolutely 110% perfect. 2. You can trust that they will be printed to the highest quality. 3. They will be produced on whatever card stock you require- pearlised, kraft, textured, etc... 4. You've got a lot more options in general and you're not restricted in any capacity- if you want gold foiling on them for instance... 5. You have the personal service and interaction that you can't get with the bigger companies that churn out cheap cards on a daily basis.
Have a look at a selection of thank you cards I have designed in the past- theres a lot of variety because they are all bespoke as it just depends upon what you want and how you want to say thank you.
I normally work with clients from the start to the finish- which are the thank you cards- but in case I haven't, don't worry, I'd be very happy to help design your personalised cards, don't be afraid to get in touch.