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Can you boil an egg? If so, you can prepare an eruv tavshilin, all by yourself. There's no need to spend clost to $4 on a kit that contians a hard boiled egg and a roll and a copy of the blessing you can find here. If you bake your own challos, then you can set aside your own roll for the eruv. If not, you can either put aside a roll from the ones you're buying to be consumed on Shabbos or just set aside a matzah along with the egg.
To cover both cooking and baking, we use a representative food for each. It's traditional to use a boiled egg for the cooked item because it's simple and inexpensive. We usually all have a spare egg around. But it's fine to also use a piece of boiled chicken, fish, or meat, as well, so long as you can put it aside where it won't be consumed until the Sabbath day. The same goes for the baked item, which can be a challah or matazh.
With upcoming holiday, we will have 3 opportunities to prepare an eruv tavshilin outside of Israel and one even in Israel, as Rosh Hashana is a 2 day holiday even there. This year, Rosh Hashana, as well as the first and last days (really Shemini Atzers) of Sukkos fall out on Thursday and Friday, the days of Yom Tov only end after the Sabbath begins. That means that in order to cook food or even to set it up to heat or to light candles for the Sabbath, we have to show that we've already begun the process and will merely be continuing. That is the purpose of the eruv tavshilin, which has to be prepared before Yom Tov begins, some time on Wednesday.
Note: The eruv tavshilin only allows cooking for the Sabbath on Yom Tov, it does not allolw cooking from one day of the holiday to the next (i.e. Thursday for Friday), though it is permissible to cook more on Thursday than will be finished that day, and then consume the leftovers on the next day