WordPress core committer & plugin developer. Co-organizer WordPress Sydney meetup group. Organizer of WordCamp Sydney 2014, 2016 and 2018.
Conference and event organisation can seem a daunting task to the first timer. Let me tell you it's a big ask for the seasoned professional too. I can help you put in place a plan to cover the essentials you need to pull off a great event.
Complex web projects require clarity of architecture from the start otherwise the project can come crashing down around you. I can give you advice on choosing the best technologies to pair with your WordPress project to help minimise the risks.
Having to change payment gateways, membership architecture, subscription models or product categorisation mid-project is going to cost you a lot of time and money. I can help you devise a strategy for your ecommerce based WordPress project to help you minimise risks at a later stage.
You can't embed Shopify into WordPress. They used to have a plugin but that has been scrapped.
Currently the best way to have a WordPress ecommerce store that sells physical products is to use the free WooCommerce plugin from WooThemes.
It allows you to receive payments from PayPal.
As you will be shipping products you will need a shipping plugin - this will depend on which postal service or courier you are using to ship, e.g. Australia Post or UPS. These shipping plugins are not free. Have a look at the official list here https://woocommerce.com/product-category/woocommerce-extensions/shipping-methods/
That's probably the minimal set up you need to start to sell your T-Shirts and hats.
There are of course a lot more WooCommerce extensions that will enhance your store and your customer experience.
Hope that helps.
If you have a bog standard ecommerce store and don't need to do anything fancy or stand out of the crowd now or in the next 5 years then Shopify is a good choice for you.
If you've seen something on another ecommerce site that Shopify doesn't have and you think you may want to grow your business to use it then I would seriously look at investing in WordPress with WooCommerce now. It is not easy to move from boxed solutions like shopify to something else.