About declaring services paid online for tax purposes

I have a small side thing, I'm developing a videogame and I have a small publisher deal that is already paying out, and I will hopefully start seeing some money come in once I release around December.

Because of that I'm going to declare taxes by myself for the first time, and one thing I'd like to know is:

I'm commissioning a few things (musics and the game's cover art) to people who are not in Japan and whom I won't personally meet. How should I pay them, and what would I then need to declare these as business expenses?

Like, would paying someone through PayPal be normally declarable, or do I need some further receipt?

Thanks in advance.