Almost everyone you ask will probably have a different way of figuring S&H. I usually charge $2.75 initially for an auction win, and will combine several more items (depending on size & weight) for that charge which includes the cost of a bubble envelope. After about 4 items or so (I usually sell bead kits when tend to be bulkier and heavier), I upgrade to Priority - based on total weight plus about $1 for packing and the associated expenses of mailing, as the Priority boxes/envelopes are provided at no extra charge by the USPO.
I also give my customers the option of paying the extra $$ for Priority and/or Insurance up front, even for one item. Some people prefer Priority and/or Insurance even for a small item. So I give them the choice.
Since my website is a PayPal shopping cart, it's a little more difficult to add in postage. PayPal only has 2 options - either a $ amount per item or a % of total sales. % of total sales doesn't seem to have a relationship to actual S&H cost - which is generally based on weight. So I usually charge a set rate - typically $1 per item - unless the item is unusually large or heavy. If a customer buys 4+ items I automatically upgrade the shipping to Priority. Sometimes I lose $$ this way, but I think it evens out in the long run.
You seem to be a very nice young man and your mom is blessed to have your help. You are also very open to suggestions... which is refreshing! Best wishes for the success of your mom's online business.