UPDATED：
Hah！！！！I success！！！！I success buying this bundle！！！！I just pay the bill directly with my debit card by paypal, without logging my paypal account, and it simply works！！！OMG I am going to create my game now ！！！！ 😎
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Hi Krishna, thank you for your patient reply. At least your response efficiency is much higher than that of the itch.io staff.
I had previously contacted PayPal customer service to inform them of the issue I encountered. However, they believed it wasn't their responsibility because they assured me that my PayPal account was functioning normally, and their system couldn't even retrieve my orders (this explanation is reasonable because I needed to link a new credit card before making the payment; without linking the card, the entire process couldn't be considered a payment process). They explicitly told me that it might be up to the merchant to decide whether a credit card must be linked for payment and suggested that I contact itch's company. So I emailed support@itch.io, describing the problem I encountered, but five days passed without any staff replying to me.
In my email to itch, I mentioned the phenomenon of being able to successfully pay for a $30 order but unable to pay for a $49.9 order, and I asked whether itch required linking a credit card to PayPal for payments exceeding a certain amount. However, I then attempted to pay at several different devs and found out that it wasn't that I couldn't pay $49.9 for an order but that I couldn't pay for the resources you published, even if the order amount was only $1 (however, just a few days ago, I had successfully purchased another developer's $1 resource).
I couldn't purchase by adding balance to PayPal because PayPal accounts in China disables the function to add balance. Sigh, maybe I really should take the time to apply for a credit card, or I can buy the assets on Unity Assets Store. Thank you for your reply, anyway.