You did integrate the shop with an iframe? Really old-school 
I don’t know WIX completely, but you should also be able to integrate the shop with the corresponding javascript code, directly - without iframe. Like discussed here: Item Detail on Mobile is HUGE
Anyway, inspecting your page showed this page as your spreadshop holder: https://www-melo-graphics.filesusr.com/html/d0265d_6519fcf472b787130fc7ab0a3fb5753d.html
If you can edit that page - which shows the code you could also use directly for wix - as said above:
Old
<!DOCTYPE html>
<div id="myShop">
<a href="https://shop.spreadshirt.com/funny-as-shirt">funny-as-shirt</a>
</div>
<script>
var spread_shop_config = {
shopName: 'funny-as-shirt',
locale: 'us_US',
prefix: 'https://shop.spreadshirt.com',
baseId: 'myShop'
};
</script>
<script type="text/javascript"
src="https://shop.spreadshirt.com/shopfiles/shopclient/shopclient.nocache.js">
</script>
change that page to
New
<!DOCTYPE html>
<html>
<head></head>
<style>
#sprd-header-container .sprd-header__burgerbutton,#sprd-header-container .sprd-header__image {display:none}
</style>
<body>
<div id="myShop">
<a href="https://shop.spreadshirt.com/funny-as-shirt">funny-as-shirt</a>
</div>
<script>
var spread_shop_config = {
shopName: 'funny-as-shirt',
locale: 'us_US',
prefix: 'https://shop.spreadshirt.com',
baseId: 'myShop'
};
</script>
<script type="text/javascript"
src="https://shop.spreadshirt.com/shopfiles/shopclient/shopclient.nocache.js">
</script>
</body>
</html>
If the whole header should be removed, then you will lose the basket button, too. Do you want that?
Cheers
Thimo