Boxes are heavy - if the gift would be safe in a padded envelope (or wrapped in bubble wrap inside a paper envelope), that will save you some money.
Bubble wrap is nice and light for padding. Also, a ton of plastic grocery bags make nice light padding as well (and free).
If you have to use a box, use the smallest box your items will fit inside to save on box weight. Ship wth flat embellishments or ribbon (lots of cute ideas online) rather than puffy bows, so you aren't using a bigger box trying to protect the bows (or include the bows in one little box and ask the recipient to stick them to the wrapped gifts).
Weigh your package (the PO usually has a postal scale in the lobby you can use) - sometimes it would be cheaper to ship with the priority flat rate boxes than to pay the parcel post or first class rate. You can also comparison shop the different shipping companies if you know your package weight.
If using US Postal Service, their rates are a little bit cheaper if you print your postage at home (you can do this confidently if you've weighed your package).
If you are shipping to several people who live near each other or will be seeing each other, pack everything together and ship to one person, asking them to distribute the gifts to the others.
*ETA* The post office charges more for envelopes that have more than 1/4" variation in the thickness - if using an evnvelope, and you can keep the envelope from being lumpy, that will save you a little money.
This comes from years of experience shipping gifts, as well as running on an online shop. I do a *lot* of shipping!
I hope that helps!