Yes, it used to be that eBay allowed a full month to pay sellers' fees. Now it's different.
If the balance due is not paid manually before about December 15, eBay automatically debits your default account. For many people, this would be Paypal, although others may designate a checking/savings account or a credit card. Should the designated account not have enough funds in it, then eBay goes to the back-up account.
Any amount incurred AFTER November 30 is not due until January.
For some accounts, the billing date may be the 15th, in which case the debit would occur about the 31st or 1st.
The safest way to control your funds is to pay in advance, keeping a credit balance or at least a very small balance due. By making one-time payments, even weekly, you can manage your funds more efficiently and avoid surprises.
As for eBay's cut, figure on a minimum of 9% of the total (including shipping) and as much as 15% (some categories have higher FVF). Add in any fixed fees -- insertion, photos, 10-day auctions, special add-ons. If you sold $800 worth of stuff, then had $175 in shipping plus another $25 in miscellaneous fees, a total of $155 is well within the ballpark.