Compare these two definitions:
— さくせい【作成】の意味 - goo国語辞書
— はっこう【発行】の意味 - goo国語辞書
発行 is more explicit in saying that the document is not just created but also made effective, often in an official or somewhat irrevocable way.
In actual use, 作成 can imply that the document was also made effective, but you can't be sure without additional context.
In terms of 伝票, a simple distinction would be:
- 返品伝票を作成する = create a return slip
- 返品伝票を発行する = issue a return slip
For example, I can imagine an app where you click the 作成 button to create a new slip, and when you're done filling it out, click the 発行 button to print/send it. (But of course, since 発行 includes the notion of 作成, there could also be an app where a single 発行 button both creates and prints the new slip.)