different packaging materials will make a difference too. check and see if your post offers flat-rate package sizes, usually these are limited only by weight and you pay a flat price regardless within that weight range. unless you add insurance or things.
buy your shipping materials in bulk, you'll save a lot on this overhead that way. plastic bubble mailers would probably serve your shirt purposes well, don't weigh much at all, and are relatively cheap.
offer a range of shipping methods. let the customer pick whichever they want, if they're willing to wait a longer time for a better price, as long as the packagind is alright it shouldn't matter. tbh, the post will stomp a box if they want to no matter how you ship it.
take advantage of DHL service too. i just mention them since you're in germany, but if der post doesn't have a basic shipping rate (something like First Class/Standard rate) that suits you, the price may be comparable.
but..... the international postal union (or whatever its called) establishes equivalencies for international mailings. (its the oldest international trade union, afaik) there should be a similar shipping equivalent in most countries, these standards are paid for in local currency but this is converted into a common postal measuring system that allows easy transactions of international mail between customs offices (the name of that slips my mind). check out whatever the roughly 1-3 week international basic rate is. here its called First Class International, whatever that equivalent is there shouldn't cost too much. it's what 75% of your customer base will go with...
yeah, bigcartel works pretty well for most purposes. you can set the currency to whatever you'd like as well (as a whole, not as individual product unless you modify a bit of code)