When you purchase from sites that use Worldshopping, such as Buyee and Tower Records, the shipping is issued as a SEPERATE CHARGE, meaning you won't know how much you're paying to ship the product until a while AFTER you complete the initial purchase. With how expensive international shipping can get, this can present as an extreme problem for budgeting, as it makes the amount of money you are going to spend unpredictable.
It's also worth nothing that Tower Records only offers premium shipping, which, at face value, sounds nice, but that means that they quite literally have zero cheap shipping options.
Please be mindful when buying from sites that use Worldshopping!
PLEASE avoid shopping for Ado merch on sites like Amazon and Redbubble!!! All of the merchandise listed on these websites are not authorized for use from Ado or her staff. In other words, the listings on these sites are actively STEALING official art.
For official vendors: Overall, CDJapan is likely the best for official vendors, assuming you're not looking for something ultra-specific. Out of all the official stores, they have the widest variety of items for MSRP, especially because of the proxy services. The only thing they really lack is the plushies. And, since they don't use Worldshopping, they offer a wide variety of shipping options that will be presented to you up front when you make the purchase. I always recommend checking CDJapan BEFORE all other sources.
For secondhand marketplaces: I personally prefer buyee.jp over eBay, because it has a wider variety of products, and the base prices are typically not as ridiculously inflated as eBay. However it's extremely important to keep shipping costs in mind, as Buyee uses Worldshopping, meaning fees can be extremely expensive and will not be presented to you up front. Please see the Worldshopping info for more.
Absolutely, with official vendors it's always important to keep shipping costs in mind (especially when purchasing a platform that uses Worldshopping). It goes without saying that international shipping tends to be ridiculously expensive.
However, most of the need for caution lies in secondhand sources like eBay and Buyee. Be sure to only purchase from reputable sellers (ex: sellers that have more than a few sales, good reviews, etc.). While eBay has features like the Money Back Guarantee, it's better to avoid getting scammed in the first place. Additionally, be careful of counterfeit or proxy products, especially if their prices are marked up or inflated. When it comes to plushies, sellers often include a picture of the tag in their listing to show the product is legit. I'm not quite sure how to determine proxy/counterfeit when it comes to other products, but it's a good thing to keep in mind when shopping, since proxy services are extremely accessible to just about anyone because of CDJapan, and this can become an issue if you are looking for legit product and the third party seller doesn't disclose that their item is a proxy.